A character reaches for a door handle (free motion, FK), grabs it and pushes the door (contact, IK), then lets go and swings the arm (FK again). Doing all of that in one mode is awkward. IK/FK switching lets the limb change mode at the right frames.
Switching and snapping
In a rig with both an IK chain and FK controls, a switch attribute decides which one drives the limb. At the switch frame, the two systems usually disagree about where the limb is, so the limb pops. Snapping or matching solves that: it sets the FK rotations to match the IK pose, or moves the IK target to match the FK pose, before the switch.
Blending instead of switching
Some tools blend instead. An IK constraint with a strength or mix value can go from 0 (pure FK) to 1 (pure IK) over a few frames. Rive's IK constraint has a strength value, and Spine's IK constraints have a mix, both of which can be animated.
Tips
Decide switch frames during blocking. Key the frame before and after the switch. Check the arc through the switch, because a mismatch shows as a small hitch. Keep the number of switches per shot low.