A difference in Doctrine
A tank with multiple primary weapons would be equipped to target multiple enemies and destroy them simultaneously.
Essentially a brawler to the modern MBT's Spear or the IFV's short-sword.
The Brawler-Tank fits a doctrine which doesn't care about losses and wants to bring maximum firepower in as small a package as possible.
Alternately, it fits a doctrine that entails fighting wildly varied enemies without requiring the tank to be supported by more specialised vehicles/infantry.
Both of these are portrayed in the Warhammer 40,000 franchise by the Imperial Guard, which field a wide range of tanks, most of which mount a variety of turrets, sponsons and hull-mounted weaponry.
The principle tank of the Imperial guard is a Leman Russ Battle Tank, which has a large-calibre turret for anti-armour work, a forward-mounted machine-gun (technically more of an automatic grenade-launcher, but lets not get bogged down) and two manned sponsons on the sides which mount anything from flamethrowers all the way up to anti-tank lasers.
The result is a vehicle which can face virtually anything and have a weapon to deal with it.
Which is important for an army which routinely fights armies with tech-capabilities ranging from scrap-metal cars out to haunted living-metal hoverships, passing through swarms of space-locusts and flying powered-armour..
Even if the tank is outclassed or its main cannon is useless against a given target, one of its secondary weapons is probably going to do something, so a tank-battalion should bring enough of those secondaries to get the job done.
That might result in heavy losses, but the imperial guard always has reserves...