Often people chose to put the sides on first just because of length, if you add the top & bottom first the length of the sides can be quite long, since the quilt is often longer than wide adding the sides first adds to the width instead of the length. If it’s a square quilt it doesn’t matter.
Such as a quilt that is 63” x 72” and you are putting 4” borders on— if you put them on the top & bottom first you need 80” long side borders. If you put the sides on first they are only 72” & the top & bottom are 71” so 2 yards handles all 4 borders.