Same thing really...taken straight from the mot guide:

"The tyres are examined to ensure that there is no fouling with any part of the vehicle".


If they stick out, then under heavy suspension load, the arch could bite into the tyre & cause a blow-out (in it's severest form). Bit dangerous. Also over time, it the arch keeps rubbing on the tyre in general, then it will cause a groove in the tyre on the top edge, and could lead to the same result: blow-out.