Spinning reels are mainly used for finesse styles of fishing, they are great for casting very light baits, vertical jigging. They can be used for any type of fish, but a spinning reel will not be good for any type of fast fishing.
Match a good spinning reel up with a rod with a light tip spinning rod and you are set for various species. Casting small crankbaits, soft plastic and a variety of smaller lightweight baits, this setup will be extremely versatile.