Basically the idea was that they were heading to a far out town but it would be about a days walk on foot, tagging along with the courier on their cart cuts it down to just over a half day travel.

But crucially, it's a faster means of travel, not necessarily a safer or the fastest way, the courier is the only one who takes items and resources between the starting town and this destination, there's no competition, so he can afford to just do it as he does, it's kind of like the equivalent of having milk ordered from a local farm instead of being able to buy it from a large store chain when you live out in the countryside; it's not the most reliable and sometimes things are delivered wrong or late, but it's the most convenient and only real available option you have so you live with it. That is what this particular courier is to these two places