The belt for those clothes is called OBI and you tie and make ribbon with it. You can choose the color and the pattern of OBI and coordinate with KIMONO/YUKATA. In addition, even though wearing the same color and pattern of KIMONO/YUKATA and OBI, it can be seen very differently by changing how to tie the ribbon on the back.
It is easy to imagine that in old days there are not many designs of the KIMONO/YUKATA and OBI like now, so women enjoy arranging how to tie OBI. In all ages, women love fashion, don't they?