I see it as role balance
There are several roles in each pillar.
In combat,you have your tanks, skrimishers, healers, archers, blasters, and leaders.
In exploraton you have wilderness, dungeon, urban exploration plus you general sage, scout, athlete, and acrobat.
For social, you have your talker, linguist, lie detector, etc.
So there is a lot of roles in the game to fill.
But why do we talk about PC balance so much?
- Some classes could only take a few roles, severely limiting a PC's chances in the spotlight.
- Some classes could take a lot of roles, extremely broadening a PC's chances in the spotlight.
- Every campaign has different amounts of time in the 3 pillars.
- Some sessions might ignore a pillar or 2 completely.
If you spend 3 sessions herdiing nobles like cats to defend a metropolis, your combat and exploration PCs aren't in the spotlight for several hours.Then the ranger and monk say screw it and go on their own to stab bears because they are bored.
That is before you get to Mr "I'm loading my lower level spell slots with role stealing spells because those slots do bad damage at this level anyway".