A new device has its own counts and adds to the users account. The device manger to control CPU usage, times run is from one place. Tracking total results, points, runtime under one account is easier (comparing to others, etc). Over the 10+ years I have been helping WCG I have had 43 different devices. It is nice to know my total contribution, daily results, etc. If you are into getting 'badges', then grouping all your devices under the same account allows you to 'earn' badges faster.
Example: 444 years, 264 days, 8 hours of Run Time (Rank: #391)

I don't think there were 0 finds. The Water project from ~5 years ago was able to find novel properties of carbon nano tubes that were only visible when looking at huge volumes of data points

And even if the medical projects haven't all generated complete cures, I believe some have found drugs that promise to help treat them at least, and have helped give researchers large datasets to look at for analysis in the coming years. Especially as computers advance, having large datasets to comb through can be quite helpful for researchers in ways we might not see in the short/medium term

Personally, I've been with WCG for a while, and used to be *very* into micro managing everything. Now I'm happy to let it crunch in the background when my computer runs. If I lose some progress from a restart/sleep, I don't mind, as long as it still does something when the computer is idle