I believe that it is useful in a few places. cppreference.com mentions templates as one case:

Trailing return type, useful if the return type depends on argument names, such as template<class T, class U> auto add(T t, U u) -> decltype(t + u); or is complicated, such as in auto fpif(int)->int(*)(int)

The syntax also matches that of lambdas, though I’m not sure that adding another way of specifying regular functions actually makes the language more consistent, since most code still uses the old style.

Additionally, the scope of the return type matches the function meaning that you can do

auto my_class::my_function() -> iterator { /* code */ }

instead of

my_class::iterator my_class::my_function() { /* code */ }

which is kinda nice