What is the main reasons for needing a lumbar puncture?

Multiple: There are many potential reasons, but the most common are to see if there is an infection around the spinal cord/brain, rule out certain brain bleeds, to look for markers of multiple sclerosis, and to check if then pressure is normal (eg hydrocephalus).

Depends: See my recent answer. Ruling out infection, bleeding, ms, and to check pressure are the commonest reasons.