I’m torn about them. On the one hand they free up the combat design to be as wildly different from the exploration as it wants. Which can result in really creative stuff. Favorite examples are Undertale, MegaMan Battle Network series, and Tales series.
But on the other they interrupt the flow of exploration, the music, you forget where you were by the end of combat and they can be very annoying if they happen to be common or just as you’re about to leave an area. The consolation prize of growing stronger with every battle only helps so much.
I’ve come around to really liking them. In short, they vastly improve dungeons in my opinion.
Most RPGs don’t manage to create interesting battles outside of boss fights. Heck, an increasing amount of RPGs fails to create any kind of challange. However, random encounter can add another layer to dungeons: resource management. You have to plan out how to tackle fights in order to get through the dungeons with your limited items/MP - do you sacrifice more HP or do you go for your strongest attacks? How much exploration can you get in? Do you need to be extra careful and plan for stronger rare encounters? Maybe even plan around lvl up healing.
Sadly, this layer is easily removed. Overworld encounters? Just dodge everything. Adjustable encounters? Grind just enough, go heal and disable encounters. Non-challanging fights? Just use basic attacks. Healing stations? No need to plan anymore. Ideally, the dungeons provides no healing at all - especially not before encountering the boss.
If you’re interested in a game with great dungeons, I’d recommend every single Etrian Odyssey.