Used to be it was a psychological warfare tactic. Berserkers (and Vargserkers) would strip naked, work themselves into a frenzy and charge screaming at their opponent. Because if twenty or thirty screaming naked barbarians running at you doesn’t freak you out a little bit, nothing will. Or, at least that was theory.
It’s also important to remember, berserkers weren’t a replacement for infantry. The groups that employed them would have some berserkers to break their enemy’s cohesion, and normal combatants to actually finish off their foes.
If the names are to be taken literally, this apparently meant naked except for some bear or wolf skins. Which isn’t really better, but you’ll see them get mentioned in connection with werewolves occasionally because… a naked guy wearing a hollowed out wolf head as a hat and swinging an axe around is “close enough?”
A single individual working themselves into a frenzy isn’t really a threat for a disciplined opponent. But, it can be intimidating, especially for someone who isn’t mentally prepared for it.
Psychological warfare aside, it’s not a legitimate combat style. Blindly striking and thrashing is the opposite of a coherent combat form. It’s possible to get lucky with it against an untrained opponent, but, against a trained martial artist, the actual attacks are going to be very easy to predict, counter, and exploit.