This is a "no brainer" when we're just making conversation. But what about when it has to do with our own lives? Oops...when we actually have to embrace the concept - no matter what the changes may be - it's not so easy.
Now I'm not talking about plain old variety but often the forced kind of variety. Case in point - when someone is no longer able to perform a hobby or even a job because of an illness or injury, they then have to switch to some other activity or job. It becomes very awkward and is often met with resistance IF the person has not had other activities to at least dabble in while they were so immersed in the thing they love to do.
I guess this phrase goes along with the "don't put all your eggs in one basket" theory too. If all your eggs are in one place and that basket is dropped or crushed, you're left with nothing. So, for the sake of your future, it's wise to find other things to occupy your time and life. Otherwise you end up depressed and worried about trying to get into a new activity when you've never even thought about it.
Variety also helps keep you from being burned out. This applies to both work and play events. The more variety you have in each area of your life, the more you mind can engage different parts of your brain and raise your endorphins. That gives you a lift in all parts of your life. In addition, it keeps you busy and that is always useful for keeping you excited about life.
In order to prevent being caught without something to do in your life, start spreading your wings and become interested in different things in your life. This will keep you from being totally derailed if anything happens that throws you off your game for awhile (or even permanently).
Take an inventory and see how narrow your life is or isn't. Are there areas you want to or should broaden. Is your life just focused on one thing. For example, my life was all medicine when I was younger. If anything had happened that would have removed me from that field, I have no idea what would have happened to me. I would have been in a great state of despair. Fortunately, nothing happened and I learned to extend my interests. And after many years, I also developed other interests other than my primary one of running. This was beneficial since I have been out of running now for nearly 4 months and would have gone stark raving mad had there been nothing else in my life.
Take a look at your life and figure out what else you might be interested in! You won't regret it.