the short answer is mostly yes, rituals are necessary.
they set things apart from whatever else you're doing, they set things in a whole 'nother dimension in a sense.
And thats what's needed, is a whole lot of energy to open up a door, create a riff in reality (even if only your personal reality) where the being can come through. not even a spell i feel has sufficient energy to open up a doorway without a ritual to back it up.
There's a lot of ways of seeing it/ explaining "how" exactly, but what remains is that energy/ effort is needed.
Also, the ritual hopes you to focus/ narrow down who you want coming through the door by creating a like/ similar agreeable energy by association/ symbolism.
you dont want anyone to come through the door that you didn't call.