One example is that if you don't need a specific size, using the loosely defined types may be faster. For example, if you specify int32_t, but your program compiles for a 16-bit platform, then instructions operating on this variable may need to run twice, since the registers can only fit 16 bits of data.

And the other way around: If you specify int8_t, but your program runs on a 64-bit platform which only can address memory in 64 bit chunks, then you might waste 32-bits of memory for this variable, and addressing it can be slower since the program has to find the 8 interesting bits in a 64-bit register, and discard the rest of the bits so they don't affect calculations.

The only reason to use specific sizes IMO is if you work with loads of data and need to pack it efficiently, or work with bit-specified protocols like TCP packets or files. If you need _at least_ 16 bit of data for example, then you can use the int_least_16_t, which may be 64-bit or 16-bit depending on the architecture.