General, overall flexibility is important to help prevent injuries like muscle strains and tears. The better the range of motion is in a joint, then the more you can do with it. Flexibile muscles are also relaxed muscles. This, in turn, can help with some speed. Now, as far as being able to do the splits, this is not entirely necessary. However, if you can kick high and accurate, then your low kicks will be accurate as well. Just about every professional athlete does some kind of stretching routine that is specific to their activity. This is what is most important when it comes to stretching. You want to do the routine that corresponds the most to what you do, activity wise. This is referred to as specificity.