OK: you're riding your bike, throttle down, enjoying the curves and suddenly you realise that it would be so much better if your house keys weren't digging into your pants. Which is when you understand why you need a bag on your bike.
A bag for your bike might seem excessive if you're only going to be carrying personal items on everyday trips, for this you have the common use back packs...no? No. We all know that just any old bag won't do: if they're not perfectly designed for wearing on the bike, like those from Givi or Bagster, you'll immediately notice how bad they fit. It's not much fun to ride around with the feeling that everything is going to fly off...and not just for the sensations on the bike, but for losing your stuff along the way.
Going to work with a small back pack will greatly help things... and also you'll have a place to keep your personal items away from the light-fingered. Remember they come in all colours, including those easiest to match .
To go to the gym, you just need a back pack that fits all your clothes. And if you like to practise sport on your bike, remember you don't have to stop to hydrate yourself if you have a camelbag, geared up in the purest MotoGP style.