since getting Shimano spd pedals and shoes in 92 I think, I cant go back.
When touring with lots of weight on a bike, clipless pedals make climbing easier (I have found that you can climb in one gear higher vs regular pedals) and of course you dont have to mess with tightening and loosening a strap, so safer as well. Overall, just improves pedalling efficiency as already stated.

My first ones were some Shimano spd model, not the top one, the one below, cost $80 or something. I am seriously impressed with their build quality and engineering as these same 18 yr old pedals are still in use by my wife. This summer she decided to try clipless, got some shoes and I passed on these oldies that amazingly enough are still turning well, not too much pull-push movement and not crunchy by any means. I bought some new 520s--$50 how can you go wrong?
My old ones were never used for serious mtn biking, ie, mud et al, so were nearly always on the road, touring and commuting. I kept them clean, lubricated, clean them after rainy days by pushing a rag through and back and forthing to get the gunk off the springs etc. But were never taken apart and insides never regreased--yes they are at the end of their lives, very easy to click out of (perfect for my wife, who was unsure she would take to clipless) but I have had regular pedals get all crappy bearing wise in much less time, even with teardowns-and even if they were probably so so quality pedals, the longevity of the old shimanos is impressive.