Despite what other people might say, it usually has to do with wear and less with overall traction. Yes, we all know that typically a greater contact patch seems to provide more traction, but it certainly isn't on the same order of magnitude as wear reduction. PLUS, technically with the same material on the floor, your static resistance should be the same anyways.
Typically teams will put wider wheels in the middle of their drivetrain where there is constant contact with the floor, typically for WCD drop centers. See 1678 in 2014 as an example.
