On Calmac ferries it varies but the main ferries you certainly have to book. Passenger numbers are restricted and if not booked you risk not getting on. For cars you will certainly not get on in summer and may have a long wait.
When I was bookng Ardrossan to Arran, one or two of the ferries were showing as no bicycle availablity so was pleased I booked and be certain rather than to turn up and possibly having to wait for another ferry.
I think I would get away with it with P&O as freight and cars is really their business and there are only ever a few foot passengers or cyclists but I still feel happier knowing I have it booked.