Per the Wikipedia article:

It is a common myth in the United States that DST was first implemented for the benefit of farmers.[38][39][40] In reality, farmers have been one of the strongest lobbying groups against DST since it was first implemented.[38][39][40] The factors that influence farming schedules, such as morning dew and dairy cattle’s readiness to be milked, are ultimately dictated by the sun, so the clock change introduces unnecessary challenges.[38][40][41]

TL:DR for the rest of the history and rationale: Most of this was rich people wanting more time for their personal hobbies, other people’s sleep and health be damned.