The purposes are many but hinge around the desire to get the very early morning daylight that occurs during the summer months shifted to the evening hours when we are awake to enjoy it . I know that disregards the early birds.
There are two reasons that are most commonly used. One is that Benjamin Franklin came up with it for children because of nighttime and that they were outside, and the hour change helped that so it was lighter or darker outside. Another reason which I'm really not sure what it has to do with it is that it is supposed to save coal.
The reason that we have daylight saving time is to maximize length of the working day in industrial revolution factories. They were usually tall (the stationary power supply was in the basement) and the ambient light came mostly from the windows. As the day got longer the hour was shifted so that daylight wasn't "wasted" on the workers after the work day was over, and artificial light was minimized in the "Dark Satanic Mills" Please note, no farmer ever cared about the time... if it was light enough to see what he was doing, he was in the field.