Covers do help protect your bike, but they're only marginally better than nothing. This includes tarpaulins and dedicated bike covers as well as BBQ covers.
A cover will reduce dew and rain and dust landing on the bike, but in turn they can sweat and build up condensation underneath. At the same time, a cover limits airflow.
The covered bike is outside so subject to temperature swings across the day/night. This also contributes to moisture especially around dawn.
So your bike will be damp and still for long periods. This encourages rust.
Ideally your bikes would be inside, in a warm dry space. Remember you can use all sorts of corners to store a bike, or use a lift to haul it up out of the way.
If that's not feasible, a closed garage or shed is better than under a cover.
However, a cover is still better than leaving the bike out completely unprotected.
Other features to consider are
- wind (do you need to tie the cover down lest it end up blown down the street?)
- security (locking the bike to an immovable object then cover it)
- noise (if the cover crackles and flexes, will it keep sleepers awake)
- drainage (put the covered bike in a place that won't develop puddles)