[onei]

Symbols in Ruby are meant to be more performant that strings iirc. If I have symbol :a, then it's allocated once regardless of how many time it appears. As opposed to "a" which is reallocated every time.

I guess it's similar to Python having a single instance of small integers. PlayStation also experimented with caching small floats which gave them some perf improvements too, but I think wasn't as performant in all cases.

[justincormack]

Lua (and some other languages) intern strings, so all strings that are the same point to the same string instance. This gives the same benefits (plus string equality is just pointer equality) without a different type.

[pawelduda]

There is a caveat in older Ruby versions that they aren't garbage collected, so they shouldn't be used for things like user input. Not a problem since 2.2 though.