My primary question, as an Android developer who can write decent enough Swift when needed and who does not develop on any Windows platforms, is why should I learn Xamarin?
The possible savings of the value of your time? If you can share even a small bit of code between Android and iOS, that is time you save in rewriting Java code to Swift or vice versa.
I can't tell you where that bar is, but especially if you are a solo developer it can be easy to undervalue your time spent porting your app between platforms.
Beyond that too, even though you don't develop on Windows now, using a cross-platform toolkit at least provides you the option. You might be surprised how many users [1] you can pick up by making a Windows version, and if it is as easy as add a couple more platform specific files to a mostly shared codebase you might get extra users for quite cheap.
[1] Windows phones have dropped to a low percentage, but that low percentage is still countable in millions of devices. Plus, the UWP continues to expand in number of tablet devices and desktops that support it, not to mention Xbox Ones and AR/VR devices.
It depends on what your career objectives are. If you intend to found a start-up you can of course choose your technology stack and ignore Xamarin completely if you wish.
However if Xamarin takes off in the way I hope it will and becomes a high-quality product under the auspices of Microsoft, I believe it will become a vital skill in the marketplace.
And then, it would be like not knowing React or Angular as a web app engineer. Depending on the company's technology choices, that might not matter, or it might mean you don't get the job.
I can't tell you where that bar is, but especially if you are a solo developer it can be easy to undervalue your time spent porting your app between platforms.
Beyond that too, even though you don't develop on Windows now, using a cross-platform toolkit at least provides you the option. You might be surprised how many users [1] you can pick up by making a Windows version, and if it is as easy as add a couple more platform specific files to a mostly shared codebase you might get extra users for quite cheap.
[1] Windows phones have dropped to a low percentage, but that low percentage is still countable in millions of devices. Plus, the UWP continues to expand in number of tablet devices and desktops that support it, not to mention Xbox Ones and AR/VR devices.