It's easy to overlook the flag when you are moving out. There are so many details which take precedence. If you can negotiate with the land owner or real estate company, you can get some ad space to let people know of your new location. The attraction does well for you and helps draw attention to the building and prospective new tenants for the land owner to move in a new business.
Keeping a flag on the pole, is the same as picking up the trash, cutting the lawn, and trimming the bushes. A well kept exterior shows the surrounding business community, this property will not be vacant long. If the lot is a part of a shared space, cars, activity, and flags all contribute to a company seeing themselves in the space.
Get a flag on the pole and show people you are open for business or open for lease.