Well, when i first got to that scene, i just tought it was her way to show me something special, since i romanced her, before the final fight. But then, after the second time (with no romance) she did it again.
This is what comes up from a search on masseffect3 wiki:
Regardless of whether a romance is pursued with Liara,
before the final battle in London,
Liara gives Shepard a gift: joining her consciousness with Shepard's
to share an intimate moment,
a practice that is normally reserved for good friends or a bondmate.
Also i remember reading on bioware's forum (sorry i don't have that link anymore, if i'll find it i'll update) that it also served as a sort of time-capsule, on which Liara "saves" Shepard's memories for future generations (this makes sense too, since the scene's background changes if you're renegate/paragon oriented, so it fits your character)