Re: the purpose of suburi in aikido?
It's not for arm power, it's for hip power. Staying relaxed, with good posture, projecting energy into the tip, grounding with your feet, turning your hip into the strike -- it's really hard to do all this together, let alone with any speed.
Working on this by yourself is very useful. Think about shihonage -- it's very close to the same motion. The same principles apply all over the place in techniques.
The other challenge that I find with suburi is being able to focus and concentrate. It's easy to start drifting off and daydreaming. Keeping your mind on the suburi is good training for staying in the moment when executing techniques.