Visiting a location in person can yield information that can't be obtained from searching records or reading through discovery.
At crime or injury locations I can discover vantage points, distances, and other factors; take photos and measurements; and compare it all with law enforcement reports. Then canvass the area for any un-tapped witnesses or surveillance cameras that may exist nearby.
When looking for assets, I'll visit a business or home to view property, take pictures of items, inquire with people, and get as much information as possible before being asked to leave.
Other reasons to investigate in the field include interviewing witnesses in person to see how they might present to a jury, and undercover pretexts for gathering leads and information.