|mileage marker on the C & O Canal Towpath trail|
The most important reason for all those mileage markers is for emergency services. Say someone has a heart attack on the trail, or you discover a forest fire. The mileage markers help you to identify exactly where you are so emergency personnel can easily and quickly locate you and/or the incident.
So the next time you're on the trail, take note of those mileage markers. Sometimes they're every mile and other times every half, quarter, or even tenth of a mile. You may find them on a signpost beside the trail (such as on the C & O Canal Towpath trail in Maryland) or set in concrete in the middle of the trail. (Such as on the St. Vrain Greenway trail in Colorado.)
Enjoy the trail, but always be aware of your surroundings.