I could go on for ages answering this, but it's late so I'll just give the Reader's Digest version.
Why does this add to my sense of realism? Simply put, because reality happens sequentially. Time only moves in one* direction. Therefore realism suggests you do things at the table in a sequence based on the time sequence in the fiction: you choose your gear first because that is what happens first, and then you attempt the score. (and before any of that you do your research/casing/etc. to inform your gear choices, among other things)
Playing through the score first and then blaming failure post-hoc on choice of gear is unrealistic and inauthentic for two reasons: first, there's no way of knowing what the player/character would actually have chosen (as opposed to whatever the failure result said she didn't have) had she been able to do her own choosing; and second, because it goes backward in time in the fiction and puts the effect before the cause.
* - if the particular game/campaign has established that its fictional time behaves abnormally then all bets are off; but such campaigns are rather uncommon I think.
If the above makes no sense, let me know and I'll try again when I'm more awake.