Besides the 2 bags you get for free, the ability to rebook whenever you want and pay the difference / get a credit is so incredibly useful. I am not super well-versed on airline policies, but last time I looked, I think they were the only US airline that did this (at least for the economy class level of tickets).
One neat trick: you can rebook your exact same itinerary if you see it cheaper and get the difference as a credit (for the Wanna Get Away class fares; if you booked with the pricier ones, you'll get it back as a refund to your credit card).
Spirit just dropped change fees entirely, and the other 6 biggest airlines have dropped change fees except for their basic economy equivalents (normal economy is changeable for free.) But for checked bags they all still really want you to pay $95 annually for their cobranded credit cards.
Now that Spirit dropped them, I wouldn’t be surprised if change fees get scrapped across the board soon given the upcoming requirement to advertise them upfront.
One neat trick: you can rebook your exact same itinerary if you see it cheaper and get the difference as a credit (for the Wanna Get Away class fares; if you booked with the pricier ones, you'll get it back as a refund to your credit card).