You can find the distance through various routes from various places in the Roman world at different times of the year. So, say you want to find the fastest route to Rome from Londinium. It would give you that. Or if you want to find the cheapest route, etc. etc. you choose that and it shows you. It also provides the cost of each route, of shipping goods along each route, etc. etc. You can change the method of travel (Horse, wagon, foot, horse relay, civilian vessel, military march, etc.) It's incredibly useful.