Simple... A dirty MAF will typically report less air than there actually is, causing a lean condition as severe as 20%. The computer will notice that it's running lean based on the O2 readings and will alter the long-term fuel trim to correct for this. When you clean the MAF, the ECU doesn't know this right away and will run too rich for some time because it has already adjusted to the dirty MAF's skewed signal. If you don't reset the ECU, it will take longer for the computer to adjust, and you could experience rough idling and have other drivability issues in the mean time.
