It had good departure times both mornings and evenings.

And it was cheap, so you could book a next day flight without paying multiples in premium.

It was fun and affordable to fly out of state in the morning, spend a day exploring another place and get back at night.

For clarity, absolutely nobody did or does this. Spirit is bottom of the barrel cheap - it made flying accessible for many people, who otherwise would not fly (think Ryan air). Absolutely nobody was interstate day tripping, especially on spirt, besides this poster.