windows has only a limited number of officially defined screen sizes, and most drivers are limited to those. If they get commands for a different screen size that is not supported by the display driver, then they choose the next larger one and fill the difference with the black border.
theoretically you can eliminate those borders by giving your game the same screen size than your monitor, but that has a lot of problems:
1) the engine has problems handling maps if the screen size is not a multiple of the grid size. So you have to use 1056 instead of 1080 and so on.
2) different players have different monitors, so a screen size that eliminates the border for one computer/monitor will still display those borders on other monitors
3) the engine has problems if the internal screen size of the project is larger than the physical monitor screen size. so you usually want to keep the games screen size lower to make more people be able to play it.
That plugin also adds an "Exit" option, so players can still exit your game, even if they don't know about Alt+F4 (Windows and Ubuntu) or CMD+Q (macOS) or whatever shortcut their Linux distro of choice uses.
Unfortunately, the plugin doesn't work with browsers (yet).