Ombi is just an interface for your users to submit requests for new media (and report issues with existing media).

Users can search for media, getting results from thetvdb/themoviedb and just click 'request'. If they have the appropriate permissions, it'll just get sent directly to sonarr/radarr to be grabbed as if you'd added them manually. If they don't have 'auto approve' permissions, it'll sent you a notification though whatever means you've configured saying 'user x has requested y. This request requires manual approval' before sending it off to be grabbed. It'll even notify the user when something they've requested is ready to watch.