The Regional Road system is merely the legacy of the former County Road system. Country roads were the most important ones of the time - Mississauga Road (County #1, later transferred to the brand new Erin Mills Parkway) connected Port Credit to Streetsville to Caledon, #3, Britannia, connected Streetsville to Highway 10, etc. Winston Churchill (#19) is a county/regional road only where it touches Halton County/Region.
What is now Mississauga had more provincial highways (2, 5, 10, 122) than Brampton (7, 10) and Caledon on a per kilometre basis. Up to the 1990s, the highways within urban areas were downloaded to the cities (Hurontario/Main Street, Dundas, Lakeshore but still signed as provincial routes, but the Harris downloads (50, 7, 136, 24) ended up going to the region for some reason. Oh, and Peel does fund paratransit.
Since regionalization, though, is there a need for the regional road system, especially when many don't cross municipal lines? Caledon can afford to look after their road system.