There are numerous, technical reasons to change buttons around.
You take tap jump off for a technical reason, for example, so that you do not jump in the middle of possible tilts.
On the other hand you can create a more efficient control set up to streamline your inputs.
The GC control by default has two pairs of buttons with the same macro -
or
for shielding and
or
for jumping.
If you hold the controller as intended, your index fingers rest on the shoulder buttons and your left thumb on the analog stick and your right thumb on either
,
, or
.
Now, if you want to nair or fair out of shield, notice that you have to hold one of the shoulder buttons with an index finger to shield, then press
or
with your right thumb and then press
with your right thumb.
Notice how quickly your thumb must be to create this maneuver.
Now, place shield on
and jump on
and keep the attack button
.
If you want to nair out of shield you use your left index to shield, your right index to jump, and your right thumb to nair on
.
You minimize the time and things your right thumb has to do and you have smoother play.
It might seem silly, but in competitive play, the player who can execute faster will always have the advantage and simple control tweaks can do that.
tl;Dr set shield to L
, jump to R
, and you can nair out of shield easier.