The majority of combinational circuits are arithmetic units that perform digital signal processing. Doing binary arithmetic in hardware is incredibly simple: You map the long addition and long multiplication directly to hardware. This works very nicely for small adders and multipliers. But as the number of bits increases, speed degrades very quickly. Since large word length arithmetic will form the critical path of most pipelines, we have to do something about the worsening delay. We have to get smart about doing maths.