Sweet's complaint is that crack has done more damage to Grove Street than the Ballas have, and he takes Carl and Ryder out to prove it. The first stop is B Dup's place, looking for him and Big Bear — two of their own, both now customers rather than members — and the visit goes nowhere.
So the three of them go after the supply instead: find the dealer working the neighbourhood, beat him, then take the crack house he works out of apart. It is the first mission in the game with no vehicle in it at all, and the first time Carl's own neighbourhood is the thing being fought over.