Why does the party need to be balanced?

[NNescio]

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.

Mutazoia is talking about 3.X, where War Cleric was basically "Fighter, but Better", even without too much optimization (provided, of course, that the Cleric takes the obvious buff spells.). One who also happens to be a full caster.

(That's why the word Clericzilla got thrown around a lot.)

Of course the 3.X Cleric can also choose to play a more supporting role, but anyone who choose War (+ some other domain) Cleric will likely wade into melee (or sometimes shoot as an archer, but there are better domains for that) because that's what the domain is built for.

Balance is far tighter in 5e. Clerics in particular got nerfed a lot, with most buff spells requiring concentration. The number of spell slots has also been sharply curtailed, while the number of expected encounters (per day) has gone up.

(And personally IMHO they went a bit overboard, with frontline Cleric archetypes having really anemic melee options despite being, well, frontline archetypes. Only way to 'fix' it is to take SCAGtrips.)