"Nowadays your video driver suite, be it for AMD or NVidia offers plenty of options to create custom resolutions. I’m on Win 10 (Pro) and have an AMD R9 280 3GB running on Crimson ReLive Software version 17.7. Once I startup my AMD interface, under the tab "Display" I can make custom resolutions on the fly. Here my 1920x900 Example:
I just click “create” (pic01) and I get a fresh template based upon my native resolution of 1920x1200 (pic02). Then I change “Vertical Resolution (px)” from 1200 to 900 (pic03) and also “V.Timing Display” from 1200 to 900 (pic04). The rest of the options I don’t mess with. Then I save, that will make the screen turn black for a second and after that the new (custom) resolution is integrated into my entire system. I can even find it under the windows Display settings as if it were a legit resolution (pic06).
AN IMPORTANT NOTE THOUGH!
I have “GPU Scaling” enabled and my Scaling mode is set to “Centre” (pic05) otherwise the resolution will spread over my entire screen.

Certain games can really benifit from a custom resolution as it's (sometimes) possible to increase the FOV by forcing it in a (ultra) wide(r) screen resolution.
Examples:
RE ORC - FOV comparison on custom (Ultra) wide(r) screen resolutions.
https://steamuserimages-a.akamaihd.net/ugc/945076402246429331/1C3626829DAB9D8104D4028761C8301C3A44DBF6/
The Division FOV comparison on custom (Ultra) wide(r) screen resolutions.
https://steamuserimages-a.akamaihd.net/ugc/945076402246505745/7890A73008F2F959E3FE405930BAAFC850C5B157/
RE4HD FOV Comparison
https://steamuserimages-a.akamaihd.net/ugc/689397056110184510/18F2404AA750B25C207DE0AF11A41A00FDB6E993/
"