I always wonder: why do some people like to do this (spreading)? I wonder the same about people who are "distro tourists". The latter tend to spend a lot of time on what seems like unproductive diddling (desktop skins, etc).
1. Because I like engineering more than programming. If a problem is best solved by, say, a mechanical system then I will do that instead. But even where programming provides the best solution, not all ecosystems are equal. If I have a problem best solved by, say, AI/ML, it is often impractical to avoid Python. Likewise, if I have a problem best solved in the web domain, it is often impractical to avoid Javsacript. SQL where databases are the best solution. So on and so forth.
2. Because I like to learn. In my younger days, I found a lot of value in exploring the different ways people live. In fairness, I've toured the programming world enough by now that I am less compelled to keep go on "programming language vacations" – at some point you start to feel like you've seen it all, but believe I would be a far worse programmer today had I not been able to take ideas from a wide range of different cultures over the years. There are some good ideas out there that don't seem to ever gain mass adoption outside of their originating ecosystem.
Diff langs have their strengths and weaknesses, the same reason you don't build every object out of concrete "because it's tough" you don't build every service out of Python (because it's slow).
Because there's so much to learn from having different perspectives.
Back in 2006, I was working mainly in C with a bit of C++. One of the things I was taught was that "macros are evil", period. Then, in my spare time, I decided to try Common Lisp. I had a blast. I never wrote anything more useful than a half-assed catalog for my MP3 collection, but I learned a lot. My main takeaway was the power of metaprogramming -- with all of its footguns and pitfalls -- and took that knowledge with me to my day job, where I suddenly had a more nuanced view of how preprocessor macros can avoid being evil.
Later, when I changed jobs, I went back to Java, which was my main language before the C stint. I slipped back into the comfort of having my memory managed for me and the expressiveness of OOP. But in my spare time, I discovered Self and Io, and the fact that you can have OOP without classes blew my mind.
At that same job, I undertook the task to make our proprietary in-house language not only transpile to C++, but also compile to be executed on JVM. Understanding how JVM bytecode instructions work was easier than it might have been had I not dabbled in Forth in my spare time.
These days my day job involves working with C++ full time, but my hobby projects in Rust taught me to structure my thinking about the ownership and lifecycle of memory allocations.
So yeah, most of what I do in my spare time with other languages is largely "unproductive diddling" if you only look at the code I write in those languages, but the insights I take away with me are useful.
I can think of many reasons. Seeing other language perspectives definitely can help with your primary language. I have experienced this. But, I have also explored other languages as a means of procrastination or ways to do something easy when bumping into the more challenging depths of my language of choice.
