You're confusing time zones with daylight savings. Time zones exist so that the same part of the solar day has approximately the same time associated with it wherever you are in the world (prior to the railroads, that was the only time - local time). So I can say "it's 4am in China" and you'll know it's the middle of the night without having to consult Google. They can also achieve the opposite, keeping neighbours on the same time when their solar days are actually different, e.g. within China. Sometimes that's useful.

Daylight savings exist for saving power (and were introduced in the war to do just this), providing more daylight hours for school children, and longer days for agriculture (now historic). There is debate about whether any of those reasons are actually valid.