One reason is to cover restoration/maintenance costs but the other would be for $$$ profit. These monetarists don't go around rebuilding things out of the kindness of their hearts. If you're going to invest in going to the middle of nowhere to establish transportation you're going to want to make bank long term, not just break even. Especially if it's a useful service for adventurers and you have zero competition. I mean, there's a reason they're "wealthy"

The non-lore reason is to remove gil from circulation to prevent inflation.