For me, mostly only that my savings account is sadder.
My water charges a flat fee per payment, so last few years I pay for roughly the year in one go. I know I'll spend it. I don't have a choice in providers so doesn't seem to make a difference if I pay it as I go or pay once and let them work it off.
But if it's someone you can change out, I don't like to overpay because refund policies can change.
We used to have Waste Management for our trash collector. One year AmEx had an offer so I prepaid essentially a year's worth to trigger it. Conveniently they raised their rates the 20% or so that I just got at a discount

But ok whatever. But by the 3rd quarter it was egregious, they were more than double the competitor. I negotiated a "courtesy" discount but they couldn't apply ahead so I'd have to call the next quarter. Well that fell apart when I tried to get them to honor it the following quarter and long story a bit shorter, I had to cut ties with them with a $40 credit on the account. They do not refund. Period. So that's $40 bucks I literally (or ironically) threw away by prepaying.
Sad to say I still saved waaay more even with the $40 loss, and I knew that risk when I prepaid. I just really didn't think they were going scorched earth in the neighborhood that fast. I had already decided it was going to be my last pay ahead with them to try to avoid any cancellation issues, just a little late on my timing! But that's my cautionary tale for pre-payments. Just assess your situation both financially and policy wise. And even if someone has a reasonable refund policy now, they could change in the future.