Why does the party need to be balanced?

[Zuras]

Now, party balance doesn't mean you have to have a set roster of characters, but it does mean that you try not to make characters that excel at everything (Unless the entire party is built along those lines). So , if Bob makes a fighter, you don't go and make a war domain cleric and muscle him out of his job. There are plenty of options in the game, that you shouldn't have this problem.

For 5e, isn’t that particular scenario almost the opposite of muscling them out of a job, though? If Bob makes a Fighter, and Joe makes a War Cleric so Bob can use his Protection fighting style when they charge the enemy together surrounded by Spirit Guardians or warded by Bless, Bob is usually having a good time.

If Bob builds an archer but doesn’t take the archery fighting style or Sharpshooter, and Joe builds an optimized Elven Samurai archer with Sharpshooter and elven accuracy and doubles Bobs damage while using the same schtick, then you have a problem.