Why do people use earth’s motions to determine units of time?

Because before we had clocks, everybody's time system was related
to the cycle of light, dark, and the motions of the sun during the day. 

Even now that we do have machines to keep time with, our bodies still
operate on the cycle of light, dark, and the motions of the sun during
the day.

Those people who are technically inclined understand that these cycles
are actually caused by motions of the Earth.