Playing against someone much better than you sharpens your perception, reactions, everything in a way you can't get any other way. Bring it on!
dragontamer's example of Magnus Carlsen is a good one - he was in the top 1000 players in the world when he was 12, and become World Champion at 22. He's intuitively good at chess. The match he played to become World Champion the first time was fascinating because as soon as he could get to the point where the game was outside of what Vishy Anand had experience of he was pretty unstoppable. He's not someone normal players will ever beat. I think most chess players would like to play him because he's famous, but if you met someone unknown with the same level of ability most people just wouldn't play them except to learn. Few people would want to play that sort of person for fun. It just wouldn't be very enjoyable.
Eventually we all die. It's fun to play anyway.
In a less philosophical mood: standing against an overwhelming force can be exhilarating. I've felt something like that windsurfing in strong wind conditions.
Down to the specific question, losing always against someone you can't defeat is very good training for winning over others.
I don't always succeed but the pure thrill of trying to survive what is usually a virtual death sentence is something really powerful and when successful so exhilarating.
My brother-in-law talked about chess for years, I finally agreed to play him, and won both games. He's never brought it up again. It makes no sense to me.
Growing up, I was the youngest, so I generally lost at whatever games we played, and it never made me want to stop playing. When my significantly older brother came back home and stayed with us for awhile, we played chess regularly, and I lost a _lot_. But I kept playing, and when I finally got to the point where I won one out of three, it felt really good.
How much fun would I have missed out on if I hadn't kept playing?
Probably he was just diving in his next obsesion at the time.