Usually having a variable drive strength allows a chip adjust the strength based on need.
Faster signals need faster edges which can be achieved with stronger drive ability, but it requires more power. Longer bus has more capacitance so larger current is required to have sharp edges. So having a too weak drive will cause problems if the signal edges are too slow.
Slower signals can work with slower edges and don't need that much power. Shorter bus has less capacitance so it requires less current to have sharp edges. Having a too strong drive can also cause problems if the signal edges are too fast.
So it just enables a chip to work in larger operating conditions by adjusting the strength of internal buffers, so there is no need to add external hardware to adjust the strength.
