Some builders angle the rear bulkhead to reduce the volume in the cockpit just behind the seat.

It does reduce cockpit volume a bit, but IMO the main reason for angling the bulkhead behind the seat, and having that bulkhead meet the cockpit coaming, is that it makes dumping water out of the boat (during a rescue or just dumping water at the beach/landing spot) much more efficient. The angled bulkhead (preferably with the sides curved forward) 'steers' the water out of the boat and little is trapped in the 'corners'. It can make the difference between having to pump out the boat (ugh!) or not.
The better-designed commercial boats (NDK and others) have angled behind-seat bulkheads that are close to the cockpit rim.
There's usually room for an inflatable paddle float to jam in behind the seat, even with an angled bulkhead.

Getting that bulkhead forward (and angled) also gets the heavy stuff like water bags that you've packed in the compartment aft of the seat ('day hatch' or rear compartment) closer to the center of the boat, by a little bit. I know it is possible to pack water in the cockpit but that's a different discussion.