You can use wifi to request, you just won't be able to see your trip status enroute as you will be outside of coverage area. I do this often when travelling.
Some cities allow cash trips, I don't know if this has been extended to all markets yet.
Some cities equally display the fare upfront, otherwise you can get an estimate, if you deviate enroute it is recalculated. It's much cheaper than regular cabs in most cities (not always). In some places you just get a regular cab and pay the meter fare (but via your card registered to the app), e.g. Dublin, where private (non-metered/licensed) vehicles aren't permitted as cabs, so Uber integrates with the regular cab drivers.