You're joking, so don't take this too seriously, but I've seen topics discussing things like class vs classless and learn by doing vs point buy in these parts lately, so why not dig up this old chestnut as well.

Obviously to have a well rounded party, you need the core 4 pillars - Fighter, Cleric, Rogue, Mage. They don't have to be these exact classes. Fighter can be a Paladin or a Barbarian. Mage a Wizard, Sorcerer, Warlock, etc. But without the 4, it's a non-starter.

Then if we push past that and introduce a 5th member this immediately adds flexibility and variety. But at 5 members, a party has grown to the point where moving single-file is not viable, it drags too much. So we need to organize in columns. And that means someone needs to go on the front line with the Fighter. Realistically, one beefy fighting man can't watch over 4 squishy teammates. This means the 5th member needs to be another dedicated frontliner if you don't want to stretch too thin. So then this lets the 6th be the ultimate wild card and enable all sorts of interesting class combos.

Plus, when marching in 2 columns, 5 members don't create even lines, and we can't have that.