I always thought Preclearance was put in to help US carriers. They could take pax from YYZ - ORD (or ATL etc) and the aircraft and some pax would continue to Salt Lake or some other western destination that Canadian carriers couldn’t get approval for. The worst example was Eastern used to fly YYZ - BUF and on to MCO, TPA, FLL, MIA while Air Canada and others couldn’t get any more landing rights. That’s all changed now so they’ve made the fees so high for preclearance that it encourages some price conscious pax to drive to BUF etc. Its not for the convenience of Canadians.