Why is it necessary to clear all of the comp. codes fomr the vehicles computer??

Jus curious

 

With some older vehicles the codes can prevent new ones from showing. With OBD-I if there are any codes, stored or new it jumps into "limp home" mode, which usually makes the engine run full rich. This can foul spark plugs, and almost always toats the O2 sensor(s). It also takes the fuel mileage WAY down, usually by 4 to 5 mpg.