I know wearing the SMP and a dive computer is a bit redundant, but it comes in very handy for timing the surface interval. My dive computer goes into battery saver mode a few minutes after a dive. Takes a few seconds to re-activate and scroll to the correct screen. The SMP tells me the surface interval at a glance.
And, you have a helluva backup in case of failure with the dive computer. You never know if you'll need it.