One point I've seen a few times that I'll show with an example. I'm upgrading my Main Hall and there's 10 minutes left on the build, but I have to leave for work in 5 minutes. I'll be at work for 8 hours, so if my upgrade doesn't finish before I leave, my builder sits idle for the time I'm AFK.
Before, I'd be stuck spending money (diamonds) to rush production. Now, if I have a handful of instants saved up, I can rush production, and start another round of MH (or any other building upgrades). Now my builder is busy while I'm at work, and I get to keep my city development moving.
They're not supposed to be game-breaking spells. If you compare the 1-minute boost to the 5% gold, no one is complaining that 5% gold isn't good enough. But if used well, it can be a helpful item that saves players frustration in certain situations.