It's not about that, it's about avoiding copying to a function parameter. If you want a function that adds a bunch of integers, you should write it as "int sum(vector<int> const&)" rather than "int sum(vector<int>)" because the latter will copy the whole array of numbers just to sum them and delete the copy, when you could have summed the original array with no downside. Arguably it should be span<int> though now that exists.

Things I read recently makes me think that default target for stuff like that is a manky compiler that's half a step above macro assembler. AKA it's much faster if your compiler is dumb as rocks and usually not with actual compilers.