The answer is in the documentation you quoted
Computes the square root of the sum of the squares of x and y, without undue overflow or underflow at intermediate stages of the computation.
If x*x + y*y overflows, then if you carry out the calculation manually, you’ll get the wrong answer. If you use std::hypot, however, it guarantees that the intermediate calculations will not overflow.
You can see an example of this disparity here.
If you are working with numbers which you know will not overflow the relevant representation for your platform, you can happily use the naive version.