Mostly it will likely boot faster because it won't be looking for all kinds of hardware you don't have.
If you have some hardware that is not being supported/detected, then a custom kernel might help this too.
If you are running a system with limited RAM (say under 64 megs or so), then a smaller kernel will take up less of this. Note the kernel will attempt to give back memory from drivers that it doesn't use, but I'm sure it isn't perfect.
The Mandrake kernel is pretty full featured (and often includes some fancy patches as well) but there is always a possibility that you'll find some kernel feature that you will find nice-to-have that the factory kernel doesn't have.