i thought they held part of the brain and are used to attract mates. the part of the brain thats housed there is the pleasure zone. i remember a jedi cutting hers off to prevent herself from wanting another person. she was banished from her people or something like that after. they are a big deal for them
They do contain part of the brain, but I haven't seen any info regarding what areas of the brain, exactly, are located there.
EDIT: The info I have (I primarily use the d20 Star Wars RPG for lore, as authors often speculate and apply literary license to make a story more interesting) says that they are sensory organs. I can see such an extrapolation being made, but it's faulty logic. For all I know, the lekku contain their taste buds or a radar system.