why do I need to put 100 miles after repair?

If your check engine light is on it's an automatic fail at smog. Light needs be out long enough to get through smog and nothing pending such as 02 sensors or cat below threshold

You can buy a 30.00 code reader at Wal Mart for OBD-2 will be in automotive near oil filters

They cleared your computer as part of the repair. Your readiness monitors must be reset before you go for the smog test. If they're not ready (set) you will fail. One important thing is the gas gauge must be between one quarter and three quarters full for the computer to run the diagnostics on all the systems to set the readiness monitors. If the gas gauge is above the three quarter mark or below the one quarter mark the readiness monitors for the EVAP system, catalytic converter, and EGR system will not set no matter how many miles you drive. Hope that helps! Jim

Some states allow one monitor to be up , with a friends P.T. cruiser we drove around till the monitors were good enough and bought it in , we had a 30 mile window to get the sticker before the code would come back up .