Wiktionary describes a recumbent as 'A bicycle or tricycle that places the rider in a reclined posture.'
There are many reasons why you should ride a recumbent.
Fun - they are loads of fun to ride.
Comfort - a very comfortable riding position with a far greater area taking your weight rather than the
tiny area of a conventional bike saddle. There is no weight at all on your wrists and no neck strain as you don't have to lift your
head to look where you are going. More importantly for some men, no numb genitals, you can ride for hours and not experience the
pain and numbness usually associated with long rides.
Aerodynamics in the form of lower wind resistance. At speeds over 25 km/hour, 90% of your energy is used to overcome wind
resistance. Recumbent bikes and trikes have a lower wind resistance, so less is energy expelled to attain the same speed. In 1934 the
UCI banned recumbent bicycles from any of the races it sanctioned, because speed records were being smashed by recumbent riders.
Variation - a wide variety of choices, over seat (OSS) or under seat (USS) steering, high or low, bike or trike.
Novelty - a real conversation starter, people of all ages will chat, especially with a trike.
A few reasons have been suggested for not riding a recumbent (some quite flawed arguments).
Safety - car drivers will not see you. Actually, you are more in their line of sight than a normal cyclist who
tend to be above a car.
Size - trikes are quite wide. They are only a little wider than a normal cyclist.