The CPS is the main timing source for the ignition, it tells the PTCM the position of the crankshaft.
Then along with the other CPS ( Camshaft Position Sensor ), the PTCM knows the exact degree of the crankshaft.
The symptoms of a slightly bad one are engine stumbling, a totally bad one will cause the engine to die ( at any speed ) and not restart.

$40-$50 is cheap insurance, and you can carry the old one along with needed tools as a spare.
( The camshaft sensor seldom goes bad, that figures as it is the one on the front of the engine easy to get to. )