A few reasons, apparently. By all accounts, the Blu-ray release contains the full International Cut, which is way longer than the Theatrical Cut that most of us have seen. But also for some insane secrets about the making of this film.
Lifeforce is out on Blu-ray now, and in addition to being able to see every pore on Mathida May's perennially naked body, you can also see the longer cut in which the story actually makes sense, and which includes Henry Mancini's haunting original score, which was mostly removed for the American cut. The extras also convey, apparently, quite how lavish this film's budget was, and quite how chaotic its production was — and how unlikely it seemed this film would actually get made.
Meanwhile, there are weird anecdotes, including the fact that director Tobe Hooper was obsessed with making sure Mathilda May was absolutely hairless from the neck down, for all her nude scenes. Make-up artist Sandra Exelby recalls how Hooper kept "demanding that she shave May's nether regions for Hooper's inspections," and she got back at him by covering the floor with trimmed pieces of wig hair.
And, speaking of May, according to DVD Talk, "The classiest extra is a third interview piece with Mathilda May. Clear-eyed and proud of her role in the film, she shows not a glimmer of regret or insecurity. As a dancer she considers her body to be more or less a costume, something to work with. She comes off as a genuine artist-professional."