Your main strings are the longer ones that do most of the hard work; they’re also the ones that almost always break first. Cross strings are there to support the mains.
You are not confined to using one type of string in your racket. You can choose a hybrid set that combines extreme characteristics between main and cross strings.
Use a hard, durable string in the mains and a soft, comfortable string in the crosses. The best of everything.
Use a premium, highly-playable main string with an inexpensive string in the crosses. Save money.
Use a thin but extremely durable string in the mains with a highly-playable string in the crosses (durable strings are always made of either kevlar or polyester; they last but are not particularly resilient). This set promotes good spin and "bite."
Mixing strings unfortunately means buying two complete sets. However, if you own multiple rackets or you intend to restring frequently, this may not be a problem.
Some players use different tensions between mains and crosses. Warning: This usually distorts the frame.
There are lots of interesting combinations.
Basically though, mixing guages, textures, compositions and prices will enable you to customize your racket like never before. Talk to your stringer for good advice.