Most of mine were covered in the options. If I were to give a free-form answer to this as an open-ended questions, my top personal reasons would be (not necessarily in this order)...
1) I am not a "good patient"-- believe it or not, probably BECAUSE my mom is an awesome doctor (who practiced evidence-based medicine, almost to a fault). That means I question everything, want to discuss everything, and get really irritated when anyone takes a "because I told you so" attitude with me. Or worse-- "because it's standard procedure." Oh, H3LL NO. At the same time, I loathe face-to-face confrontation unless necessary. Thus-- as long as I'm low-risk, it's not an emergency blahbitty blah boilerplate disclaimer-- I NEED to be the decision-maker and be working in as much of an "unimpaired state" as possible. Not to "control" things, exactly, but not to put myself in the position of passive object if it's not an emergency. To put myself in the most respectful possible space, with professionals whose judgment I trust. FTMP, that will be homebirth for me, with MWs who are highly-educated and experienced.
2) (This is for homebirth...) Because it's the safest option for me and my baby, or, at the very least, the risk is minimally increased relative to the psychological and physiological benefits for me. I actually believe that the best studies indicate mortality is equal and morbidity less for homebirth with a qualified attendant (than for hospital birth). But I'm saying that even at WORST, studies show/will show only a tiny increase in risk, and that such a tiny increase is still palatable to me considering the rewards, the ease, the psychological comfort and physiological benefits. Basically, I don't think that additional risk exists, but I do believe that if it does, it's tiny and worth the great marginal rewards.
3) Mobility and overall... again, not more "control" but "an increased range of options." That's huge for me. Baby seems "stuck," but "almost fits?" I can much more easily move and adjust and squat, etc., vs. having to be subjected to a vacuum (and possible generous episiotomy). Baby has shoulder dystocia? Easy Gaskin vs. McRoberts, etc. Want to pop in the shower? Easy. If I have an epidural, not so much. When I was first really researching births (8 years ago?), I tried to do the "devil's advocate" thing with my mom, who had two hospital births without pain meds. She said, "Here's the bottom line. You prepare for a NCB, then at least you have options. You can always get pain meds if it's not working for you. You don't prepare? You're getting the pain meds. Would you rather have more options? Or would you rather have the decision taken from you?" Me? I'd always rather have more options.