Mary K's post shows a good example of how to set it up. I will address some of the ways I use it.
I have both an 80D and a 6D with WiFi connection, and have used it to connect to my Galaxy phone. This allows the phone to be used as a wireless remote for the camera, including showing a live view display on the phone.
It has been most useful for macro work, like shooting frog portraits close to the ground with the 6D (my 80D and your 90D make this a bit easier with the articulating screen).
Shooting at night it has been really useful to review focus, composition and exposure without attracting bugs to the camera display or risking moving the camera while reviewing.
Another use is for transferring pictures from the camera. There is an option to transfer the whole full size file, or a reduced size version.
I use this on site to share pictures with people quickly via text or email or to post to facebook.
I also use it to post or email pictures while on vacation or at our cottage where my phone is my only internet access.
It is nice to have a quick way to share some shots with people on my phone which is easier to handle and look at then the back of the camera. When my girlfriend or one of the neighbors ask "What did you see this morning?", I can hand them the phone to easily show some examples.
Another use is to back up important once in a lifetime type shots for security, to KNOW that I have certain shots backed up and saved in more than one place before I even leave a shooting site.
It can be a pretty useful feature, even if it seems fiddly to set up and use at first.
