Barns and shops need their own limits
On a landlord policy, as on a homeowners one, detached structures draw on a percentage of the main building limit. On a rental with a barn, a shop and a set of corrals that default is almost always short, and nobody compares it against what is standing until a fire does it for them.
What the tenant keeps there is a live question
Tenants with animals, equipment or a trade operating from the property change the exposure substantially. Some of that is insurable and some of it needs a different form entirely. A lease that is silent about use leaves the owner carrying an exposure they were never told about.
More land means more people on it
Visitors, deliveries, people helping the tenant. As owner you can still be drawn into a claim arising from the condition of the premises, and the liability limit on a landlord policy is usually left at the lowest option offered.