Two potential advantages are lower RAM usage, and very fast comparison because strcmp becomes a pointer comparison rather than character-by-character. The trade-off is that every string construction then requires a hash table look up.
The computer science name for this technique is string interning. Julia’s Symbol type is interned like that, and I believe I’ve seen posts about (ab)using it for general string interning – but I’m not sure if such a use-case is intended 
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See also: [ANN] InternedStrings.jl: Allocate strings once and reuse them