(Spoilers Main) GRRM's use incest for different reasons in the main series

So a lot of people rightly think that GRRM uses incest as a means of shocking sexuality and as a means of showing off the toxic family dynamics, but I think he has different purposes for each one that arent often talked about.

Craster: A Gross Practicality

As abhorrent as it is, there is a gross practicality to Craster's methods. Ill explain.

Although not necessarily made explicit, the text does show us that Craster has some sort of agreement with the Others. In return for regular sacrifice of newborn boys, he is left in peace by the Others. They may even protect him somewhat if Mance is to be believed.

The text establishes that peace with the Others is possible, but Craster's peace is not something to emulate. Craster is the intelligent cow selling his calves to humans in exchange for free range over a pasture. Selling out humanity for his own selfish desires. Sacrificing his boys and enslaving his daughters. That is what a Pact with the Others means, rape and enslavement.

But there is a practical element to Craster's incest that is necessitated by this agreement. Craster's first wife is old, likely not able to have children anymore. Or its more difficult for her to do so. The Others will only keep their agreement if Craster continues to supply children for them, after which he becomes just another Wildling. So he raises his daughters to be glorified birthing machines to uphold a pact that largely benefits him.

This is something Craster cannot realistically do with other Wildling women as easily. Wildling women need to be taken against their will and can slit the throat of men whom they hate. And pregnancy is a dangerous thing that they will not go through only to give up their babies to strangers. Craster's daughters though? Girls he has raised, conditioned and isolated so that they lack alternatives? More easily done.

In short, Craster's arrangement is almost a practical one. And a gross snapshot at what peace with the Others would look like. Women enslaved as birthing machines with no respect for their autonomy, baby boys sacrificed to a race of ice demons, and only one fat old buzzard middle man truly benefitting at all. Indeed, this probably what the Night King did on a grander scale.

Targaryen Incest: Failure to integrate, inability to change with the Game

Again there is a lot that could be said for why GRRM has the Targaryens insist on incest. The Ptolemies, the blue blood criticism, and how incest is so often linked to an inability to choose that often accompanies marriage anyway in Westeros. But I think the primary point of the Targaryen insistence on incest is to highlight their failures to integrate with the people they rule.

The truth is what the Targaryens did with Westeros is just a larger scale dragon assisted version of what each of the Great Houses once did with the territories they administer. However, unlike the Starks or Lannisters, they Targaryens failed to fully integrate with the people they ruled.

Marriages in feudal era often help facilitate closer ties. Which makes the conquered people more amenable and easier to administer. The Targaryens obsession with bloody purity is an entire pseudo magical system developed to explain why the Targaryens didn't develop strong ties with the people they conquered.

Sure they adopted certain customs. Like chivalric traditions and Faith of the 7. But the holding onto things like close-incestuous marriages highlights the Targaryens failed to truly let go of their semi-mythic status.

The Dance should have brought an end to this practically speaking. The dragons were dead, the Targaryens could no longer rely on their magical firepower keeping them at the top of Westeros' foodchain. The dragons catapulted Targaryens to such position by concentrating power they didnt need to play the Game of Thrones the way other houses do. But they couldnt let go of it, learn to play the game. And Egg's attempts to try course correct failed due to his refusal to sacrifice his children's happiness. And this failure meant that it was inevitable they would eventually be supplanted by a Great House marriage alliance block.

The Lannister Twins: Love and Hate can mate. The thing in the mirror

There are many reasons for the Lannister incest too. Tywin's own lessons twisted (a Lannister is worth more, married his cousin) further by his shitty parenting. The death of their mother when both were young. And most importantly: an exploration of what it means to both love and hate ones self.

The man looked over at the woman. "The things I do for love," he said with loathing. He gave Bran a shove.

A big theme this series explores is the contradictions of what it means to be human. How a man can only be brave whilst afraid. And what it means to reconcile these contradictions.

Cersei and Jaime both think of each other as their gender inverted mirror image. Whilst that might not technically be true, it is a lie that they sincerely believe. And in their interactions, we see what it means to both love and hate yourself. A contradiction they both struggle with. And a contradiction that will eventually lead to both their deaths.

In short, Cersei and Jaime's incest will be GRRM exploring the failure to reconcile the contradictions of ones self. And we will see the Lannister twincest come to sad and gruesome end as Jaime strangles Cersei to death. Not simply out of passion or hate for Cersei, but because Jaime cannot stand to see what is reflected back at him in Cersei's eyes. There is still a part of Jaime that hates how he ended up as the Smiling Knight rather than the Arthur Dayne he idolized.

TL;DR Its easy to dismiss GRRMs use of incest as his love of shock, taboo and the perverse. And there undoubtedly is an element of that. But I think the main examples of the incest in ASOIAF series all have different (but linked) purposes.