There are a number of problems with diving with a buddy or without.
Even when you have a buddy, as soon as you loose contact you are solo but you don't have the advantage of knowing it.
Quick quize, "How many have surfaced at the end of a dive without a buddy or with someone different than they submerged with?"
Finding a good buddy is helpful as it is much easier to stay with someone who has the same underwater goals as you. A bottom crawler with a spearfisher is a bad mix. A person who burns a tank of air within a few feet of the touchdown point is a bad match for a cruiser who swims a mile or more on a dive.
Also, how do you identify your buddy and they you?
In the days of black *everything* I used to paint my name on my hood, gloves and fins. This way when my buddy looked at me, they knew it was *me*.
There is an alternative to a buddy for some dives and practice sessions, a tender. This person doesn't have to be a diver but divers usually make better tenders and if they are a diver they can also double as a safety diver. The tender can watch the bubbles for location and movement and can monitor time. A cell phone can call rescue if it is needed. If you use surface supplied air (hooka rig) then running out of air is unlikely.
As always;
Do what you will, but
Know What You Do!
michael