In a nutshell, Tournament Variations are a set of pre-defined loadouts created by NRS themselves. Why its important to use them is that if youre online in Ranked mode or want to enter any officially affiliated tournaments, then these are the only variations you are allowed to use. NRS put this restriction in place to maintain gameplay balancing on the competitive scene, otherwise you could get custom builds that completely break or exploit certain mechanics whereas the Tournament Variations are usually very good all-around.
Source: N4G PC How and why to use Mortal Kombat 11s official tournament variations