Non online coal RR hoppers

I have noticed a few hopper cars available in model kits with road names of CN, B&M, M&St.L, Seaboard Air line.
If there is a prototype for these cars, why did the RR have them? I don't think they had online coal mines to serve so why did they own so many cars? Was it to take advantage of the back haul of coal to their territories to serve heating and other customers? Won't it have been cheaper to just haul other RR coal hoppers to your territory?

I think the manufacturers often make models available with road identification that never existed in real life. I may have an opportunity next week to check some old Equipment Registers and see if those roads actually owned any hoppers.

Keep in mind that open top hoppers are not used exclusively for coal; they can also be used for aggregate, sand and gravel, etc.

Sometimes the cars were used for company coal. I have seen pictures of the large mountains of it on the New Haven, at Cedar Hill Yard. From what I've read, most was sent by ship from Virginia, and other ports served by coal hauling roads. This would be distributed to engine terminals around the system. B&M had ports around Boston deliver coal too.

Example in support of Cowford's observation--
Som of the New Haven's last profitable freight business (ironically, making things easier for truck competition!) was aggregate for high-way construction: Connecticut Thruway, I-95, I-91. I remember from the early 1950s (I think it would have been around 1953, plus or minus) when we drove by the "White Oak" trap-rock quarry (Southington, CT) the siding would have long, LONG, strings of New Haven open-top hoppers.