Here, you can see that the facia board and gutter are still attached. However, the facia board is pulling away from the rafter tales and allowing water to enter the eaves of the building.

When the gutters and facia boards pull away past the edge of shingles, the water begins to enter the building and rot the decking on top

In this close-up, you can see the gutter has wholly detached from the facia, causing it to twist and pour water down near the foundation. Water management of any building is a crucial component of maintenance.

This is an example of what water intrusion can do to foundations when not addressed.  It can cause settling, cracking, and mold.