Optimize games: If you're like me, chances are your normal account is littered with all sorts of useful tools, many of which are always running and sucking up a bit of your CPU power. When you use fast user switching, all those tasks are suspended, and 100 percent of your CPU is available. It probably won't make a difference if you're playing Chess, but if you want to play a full-screen, high-resolution, real-time 3D adventure game, you'll want all the CPU power you can muster. So fast-user-switch to your Gaming account before starting, and that's what you'll get.
Keep work and personal activities separate: If you use one Mac for both personal and work activities, creating Work and Home accounts can help you stay on task while working, but still keep your personal files away from your work files. Set up your email, browser, etc. on each account with only those things you need for work or home, and you'll be less likely to do personal tasks while working--or to work when you're supposed to be relaxing. If you do need to do something, though, fast user switching will have you into the proper account in a couple of seconds.
Have you switched lately?
I've been using multiple accounts on my Macs for years, and find them indispensable to both my work and my play. If you haven't tried this feature out, I highly recommend doing so. There's really no downside (other than a slight increase in disk space used) to having and using multiple accounts.