You're correct, in that they want to see what the reason is for all your pain, since you have a "small" herniation at L4-5. They don't feel that is large enough to be causing all your pain, so that's why they're doing it.
Try not to worry about the procedure. I'm sure you'll do just fine. 99% of them go great. Just remember to keep your head down for a few hours afterwards!!! Otherwise you'll have the headache from HELL.

If you DO end up with the awful headache, ask them for a "bloodpatch." That will help the headache. The stupid nurse that caused mine, just gave me an injection of morphine, which didn't touch it.

God bless & take care. Hugs, Lee