A lot of school districts don't have the budget or expertise to upgrade their very old computers from the Internet Explorer + Windows 7 combination. Even if they could get Windows 10 for free or cheap, they might have custom software that only works on IE and/or Windows 7. It's very unfortunate. There's a similar situation with businesses that don't have the ability to better fund their IT departments.
Here's an example: some older versions of Sharepoint have an odd quirk where embedded pages (iframes) viewed on IE 11 get viewed in Compatibility mode for IE 10. That makes it so if you want to support users embedding your site in that view, you might have to support IE 10 in some way.
Source: I spent a month fixing IE 10 specific bugs in Microsoft Sway (and documented it on GitHub). It's fascinating work.
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