Besides the things others have said, some major innovations started as corporate-sponsored pet projects. Gmail is a commonly cited example. I believe many of 3Ms successes (including post-its) also belong to that category.
While I absolutely support the idea that employment should include professional development, let's be wary: first, of survivorship bias, and second, of perversion of incentives.
How many side projects, at Google, and then at other places, didn't turn into runaway successes?
More importantly, if you tell management "you should let us work on side projects because it could make the company millions of dollars", watch how fast "did you use your 15% time to invent anything that is marketable this week" becomes a part of your performance review, and your "side project for personal development" becomes "PMs breathing down your neck asking for progress and status".
Oh, no, I would be surprised if more than one in every 100 pet projects as much as break even.
But that's the point. Your run-of-the-mill incremental improvements with a fairly certain ROI of 0.5 % or 2 % is what you spend almost all week doing. Then you have a few hours of doing whatever you want. And a very small fraction of those things will have a ROI of thousands of percent. And you won't know when it happens until it's happened.
How many side projects, at Google, and then at other places, didn't turn into runaway successes?
More importantly, if you tell management "you should let us work on side projects because it could make the company millions of dollars", watch how fast "did you use your 15% time to invent anything that is marketable this week" becomes a part of your performance review, and your "side project for personal development" becomes "PMs breathing down your neck asking for progress and status".