The new versions of the functions are only used if LuaJIT's JIT compiler is enabled.
Those functions are often combined with ImageData:mapPixel or ImageData:setPixel, which also have LuaJIT FFI versions in 0.10.0 - so without JIT-able implementations of the math functions, the FFI code in the ImageData methods would not be JIT-compiled and would end up taking up to 10 times as long to execute compared to the non-FFI versions in 0.9.2.