The purpose is to put more of the daylight hours in the evening when people are able to enjoy it. And/or when it may reduce the use of electricity (though studies show it probably doesn't have much or any effect).
If the sun rises at 5am, few people will be awake to take advantage of that light, so shifting it to 6am has not much effect. But shifting sunset from 7PM to 8PM gives people an extra hour of daylight in the evening.
If the sun rises at 5am, few people will be awake to take advantage of that light, so shifting it to 6am has not much effect. But shifting sunset from 7PM to 8PM gives people an extra hour of daylight in the evening.