Farmers used tar because farms had many wooden and fibre objects exposed to weather, animals, mud, and wear. Tar helped protect fences, gates, tools, carts, sheds, and sometimes rope or leather gear.
A traditional farm was full of vulnerable materials. Fence posts sat in wet ground. Gates moved in rain and sun. Cart parts, tool handles, buckets, ropes, harness, sheds, and outdoor timber all needed maintenance. Tar offered a practical way to slow decay and shed water.
Pine tar and other wood tars also belonged to the visual world of rural life: dark posts, smoky sheds, tarred ropes, and treated timber. The smell could be part of farms, stables, and workshops, especially before modern paints and preservatives took over.
Older sources may also mention tar around animals, hooves, wounds, or livestock care. A modern reference site should be cautious there. Historical use is not the same as current veterinary advice, and tar products vary widely. It is safer to describe those uses historically rather than recommend them.
The farm answer is mostly about maintenance. Tar was useful because farms were outdoor working systems, and outdoor working systems constantly fight water, rot, abrasion, and time.