Some are wireless. Some measure cadence (pedal strokes per minute). But all measure speed and average speed. This is critical for getting in shape. And helpful in maintaining an appropriate pace during the ride.
The CO2 inflator was originally a favourite among racers, but is now commonplace out on the trails.
SADDLE BAG ESSENTIALS
Multi-tool, Inner tubes x 2, Tire levers x 2,
Glueless patches (mends tubes in seconds),
Chain-splitter (can be included on multi-tool)