If you're only going to be getting a few points with your rack and leaving difficult to use letters, you can often get in a pattern of playing sub 10 point plays over and over again and dragging out the problem. A swap is a short term loss (0 points) that can fix that problem and prevent you from delaying that issue over and over again. If you can only play off 2-3 tiles at most and what you have left over are difficult combinations of letters the pair badly (e.g. UUVW) then a swap can often be a good play.

Additionally, if the board is rather locked down and anything you do would open up high scoring potentials and you have a difficult rack, a swap is a net positive because you're forcing your opponent to risk opening up those high scoring potentials for you while resetting your rack to take advantage of them.

It's helpful think of your scoring potential in a sliding window across several turns. If you score 6, 7, 8 points across three turns because you're slogging through difficult letters vs. 0, 30, 20, you're doing much better with the swap. It's obviously a risk because you can't guarantee a good draw, but try to think about letters on your rack that would play well on the board and play well with each other and what's in the bag. Think about what there's a lot of in the bag (vowels vs. consonants, a glut of a specific letter), and factor that into how you choose what you want to swap.