Unless of course the pet projects happen at work during working hours, but finding a place that allows that is probably a whole different kind of beast.
Work is when someone tells me what to do or where/how to do it, which is why they usually have to pay me obscene amounts to make it happen.
I used to be a maths teacher and I love maths, and would do it for fun outside of my job. That in turn often helped with the job itself, and certainly helped to keep my interest in the subject alive even when 90% of the job was teaching fairly basic stuff that wasn't particularly exciting.
I'm now a software engineer - I'm much happier in my job, but I don't love writing code enough to ever want to be doing it outside the 40 hours a week I'm paid for. I'm slightly jealous of the people who do, because I know first-hand that it's a great way to do better at work and keep feeling fresh and interested in what you do, but forcing yourself to do it doesn't bring any of the same benefits.
To have pet projects is to break out of this daily slow death and to keep learning and being intellectually challenged while keeping up with the industry.
I've seen to many "interesting" technlogies being crammed into projects just because devs needed them on the CV.
If people can test out everything they want without having to claim that they need it on a project it can save ourselves a lot more than 15% on any project or product that lives for more than a year or two.
At work we are still sometimes suffering because of how cool Redux once was.
I would like a competent plumber and if he does some fancy work in his spare time and gets even better than great.
But I would raise an eyebrow if beside my work he will also want to be paid for 6 hours he spent in the weekend, experimenting on his bathroom...
I'd expect if the plumber worked for a plumbing company they'd probably invest some time and money into skills growth, and pay for it out of the money they charge you.
And I truly think it’s the same for your codebase.
If you do not offer the possibility to learn to your employees they will try to learn while working on a project which is where problems come from : speculative programming or hype-driven-development.
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".
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.
Beyond that, I don't care if it's cancer research, self-driving bicycles, an email client someone else at the company uses, or a script to automate some commonly performed actual work task.