Silverlight's real usage was rapid internal corporate app development. Internal tools basically. It is hard to describe exactly how absurdly popular Silverlight was at the time within corporations.
Microsoft had a big internal political battle and did the stupidest thing possible and abandoned Silveright, which pissed off a LOT of companies, who then started building internal web apps instead. This lead to a slow migration off of Microsoft's technology stack within companies, and now basically Office is what is left.
For younger developers, it is hard to describe what life was like before.
So you'd go to work, and there was a custom C#, or possibly Visual Basic, time tracking app you logged into. Internal corporate web pages were written in ASP, or ASPX. Lots of internal databases were running on SQL server or just MS Access, and you directly talked to them through custom apps running on your machine.
Because the C#/VB development experience was so good, this was honestly easier than writing a website is now. Because apps only had to run on one platform (Windows) and mobile didn't exist yet, making UIs was easy-peasy. Microsoft made a ton of inroads into corporate because Bob from Accounting figured out he could setup a database running off of a local file share (MS Access) and write a simple GUI to keep track of vacation usage using some books he found in his local library, or maybe he even got ahold of some old MSDN CDs, which literally had higher quality documentation and examples than anything made in the last 15 years or so.
Silverlight was a continuation of that linage, super easy to write UIs in, super fast to develop. You could data bind a form to a backend database in a few minutes. You'd get full accessibility, hotkeys, everything, faster than you can debug a single CSS issue.
From what I understand (I was a very low level employee when it all went down) the OS org got pissed that the developer org was basically making the dominant UI framework, ripped it away from them and rewrote it as WinRT. Short of it is, Microsoft ended up losing mobile, they lost the trust of corporations, and they lost the trust of developers. After Silverlight died there aren't really anymore "Windows developers" as a career field. Everything moved to websites.
