Because it's annoying to have to implement it every time you make a one off script (or small projects) among other things.

And it is usually a pretty basic iterator standard to have it so that someone maybe not as familiar with concepts like reduce can just sum the sequence.

Or if someone not familiar with reduce read your code, and is like "what is that `.reduce(0, +)` thing everywhere!?" while instead it should be a no brainer `.sum()` <-- "yup make sense"

https://doc.rust-lang.org/std/iter/trait.Sum.html