Well, you might not understand it, and it might not really strictly be necessary but bear in mind a few things.
~Putting a birth plan together can involve useful research so even if it is never referred to, at least you are informed about the on-the-spot decisions you need to make. Better to have thought about it, even if you change your mind.
~Preparations take people different ways - some need lists, others need wine, etc etc etc. Especially if you've been frightened half to death by all the horror some people seem to feel they must prepare you for, preparing yourself YOUR way is a useful way of taking back some feeling of control, however false.
~If your husband/partner is a doofus at remembering anything at all in stressful situations, it is useful to give him a list of idea of how to help you so when he is panicking and you are wishing he would stop asking you stupid questions, he has a refuge.
~If you are not sure your principles on birthing are going to be taken seriously by the midwives (I had one mock the idea of hypnobirthing), it is worth having a sheet you can hand over that makes your wishes absolutely clear in case you can't get into deep communication at the crucial times.
It's harmless unless you get so fixated on it you can't react sensibly to what is actually happening around you. I used the words, "as far as possible" a LOT on mine for DS1. The one for DS2 was about 1/4 of the length and mainly for dh's benefit! Interestingly, with DS1 it didn't go to plan as I was exhausted after being induced for 5 days so I changed loads of things, but not the 2 or 3 things I was absolutely sure I didn't want. With DS2 it was all great.