There are a number of places where footprints can come in very handy
Taxation: As @Mapperz said, taxation is one area. The percentage of
property that is built on is sometimes used as a tax criterion.
Planning: Knowing where structures already exist on property can
help in the planning process due to applied setbacks and minimum
distances from houses and property lines. Another example would be
planning the location of a water line or a sewer line that needs to
cross property on an easement. If the footprint exists, the location
constraints become more immediately obvious.
Public Works/Public Safety: Knowing where structures are located can
show where access to the rear of property is constrained. This can
be important for public safety, if there is a fire, can a fire truck
get to the back of a property, or is there not room between two
structures. From the public works perspective, if there is a
facility running on an easement across a piece of property, knowing
where vehicles can get in to be able to effect repairs is critical.
In some ways, these mirror the public sector uses. In a lot of cases, the public sector is creating data layers and then finds a benefit in exposing them to the public. Here are some possible reasons why.
Planning: If someone wants to put in a new building, they can perform initial research to find where existing structures are that
may constrain their plans.
Visualization/Simulation: If someone is trying to create a 3d model or fly-through of an area, doing extrusion on the building
footprints vs the entire property, will give you a more
Analysis: Line of sight analysis for future development. If you are designing a building having footprints will allows you to analyze
more accurately what will or won't be visible. This could also be
useful for property valuation for sales purposes.
Suitability analysis: Example: A solar panel contractor is evaluating an area for marketing purposes. They take building
footprints to see whether the majority of the residential structures
are oriented in a way that makes them more suitable for a solar panel
emplacement. If there is not a high percentage, it is potentially
not worth marketing in an area.
These are just a few ideas, but hopefully they give you some direction.