If you have elevated rest, most put sights on too.
Not in my experiance.
And shoot vertically
I shoot vertical or nearly so - but cant if conditions require it with no problems if you practise it.
A rest simply allows for a much more forgiving and easier to tune setup to acheive the best arrow flight consistently. Few of us are blessed with a perfect release and/or form on every shot - therefore the more forgiving your setup, the better the chance of good flight with those less than perfect shots happen.
Steve