3-partition problem: why $b/4 < x_i < b/2$?
The 3-PARTITION problem is NP-complete even without this condition. It is a stronger result that 3-PARTITION is NP-complete even with this condition. The condition presumably comes from the reduction used to prove that 3-PARTITION is NP-hard.

The fact that 3-PARTITION is NP-complete even when all integers are bounded in $(B/4,B/2)$ can be helpful when using 3-PARTITION to prove that other problems are NP-hard.