[ravi-delia]

They're largely a holdover from smalltalk and lisps, which use them as a sort of generalized token. I find them handy mostly because you can express things like slot names, enums, or keys in a way that's unambiguously not user provided. In Elixir for instance (which has Ruby's symbol syntax slapped on Erlang's atoms), you'll often report a failure with {:err, "error message"} so you can pattern match on it. In principle, with immutable strings, you could just have {"err", "error message"}, and it would work the same! But that's hard to distinguish from a list which happens to contain two strings.

Of course, the only thing prohibiting :err from being part of the data is convention. But if you're just looking over the code, the atom stands out almost like a syntactic feature, so it's easier to hold to. Plus, since they're interned strings underneath, you can use them in macros to make things like schemas unambiguously, and convert them into migrations with very little magic. So that's it, nothing you couldn't do with interned strings, variable names, and enums, but all in one handy little first class datatype.