@IrSoil already covered a lot of good reasons for having a platform-independent API.
Just for fun, I did a bit of archeology in our SCM history and this code dates back to at least UE3 days and 2011 (very likely even older).
There’s at least one example where having a platform independent API was useful, in 2015 we provided a custom atan2 implementation due to compiler bugs:
FMath::Atan2() no longer calls atan2f() because of some compiler or library bugs causing it to randomly return NaN for valid input. It now uses a high-precision minimax approximation instead, measured to be 2x faster than the stock C version.
In some cases like this one it’s useful to be able to replace a standard library implementation with something faster, e.g. if the standard library has stricter requirements on accuracy that forces them to use a slower algorithm.