Because the smaller pellet cookers have smaller volume for ash accumulation - a quick vacuuming before each cook is easy insurance.
You can even get a wet/dry vac head and hose that fits on a 5 gallon bucket from Lowe's for about $20. Be sure to put the fine paper filter on it before you vacuum ash - or you will think that you are standing over a volcano when it belches the ash cloud in your face