What function do the sex-crime statistics on each section's title page serve?
These prefacing quotes ("statistics") are technically called epigraphs, and in literature, can be meant to serve a number of functions. Authors use a epigraphs to introduce themes, to summarize ideas, to present counter-points (or points of comparison) for what they are about to present, or to set up context.
When the four epigraphs are looked at broadly, it could said that their function is to draw attention to sex crimes against women in Sweden. Given that two women in the novel are victims of sex crimes, these epigraphs serve both to foreshadow future events, but also to possibly give some credibility to the authenticity of what is otherwise a fictional tale. As statistics, I'd be hesitant to give these epigraphs much weight. This is primarily due to the fact that they are given without any source material, meaning, they could be completely made up.
Also, when you compare the third section's epigraph and the final epigraph, you might notice some striking irony:
Thirteen percent of the women in Sweden have been subjected to aggravated sexual assault outside a sexual relationship.
Ninety two percent of women in Sweden who have been subjected to sexual assault have not reported the incident to the police.
If 92% of the women who have been assaulted in Sweden are not reporting it, then where does the 13% statistic come from above? It has been suggested that Larson is writing here of a personal experience (witnessing a sex crime as a young man and leaving it unreported).
The original Swedish title of this book directly translates to "Men Who Hate Women." This title was changed in the English translation because publishers believed the original might not be as widely received.
What this information suggests to me, is that the author wished to include a very serious (and possibly personal) sub-context within this novel, without making the subject of sex crimes against women the primary plot of the story itself. In many ways, he succeeds in drawing attention to an actual societal problem, but does not alienate nor narrow his audience in the process.
The Girl With the Dragon Tattoo is centered mostly around solving the mysterious disappearance of Harriet. As the story progresses, Blomkvist unearths more and more disturbing pieces of evidence and family past, including sexual abuse. This could be one reason why Larsson decided to include the sex crime "stats" in his novel. Also, Lisbeth, one of the main protagonists, was sexually abused, which also gives reason for the stats to be put in there.
Hope this helps!