Class Levels

A Game Changing Idea
White Box class levels are the major way PCs improve. The rules list 8 levels for the cleric class, 9 levels for fighting men and 11 levels for the magic user. Upon achieving those "name" levels, human PCs can acquire baronies or their equivalent and enter another largely undefined level of play where managing estates, commanding armies and engaging in politics presumably replaces adventuring. This makes sense as White Box grew from wargames where managing estates and fighting battles are the main activities. For human characters the rules state there is no upper limit and levels may continue on indefinitely. The non-human character is limited with a hard cap on level as part of the "game balance" system.
The highest level character I currently play is 14th level. My experience in almost 40 years of gaming White Box and later editions of the game is that rarely do PCs progress much above 9th level. Many campaigns which I have been a part of have ended way before any PC reaches even that level.  For years I gamed under the impression that low-level play, basically under 5th level, was the sweet spot for the game. Everything was potentially deadly and each class relies on the others for support. I have come to appreciate the nuances of higher level play, but I still don't see much advantage gained beyond 10th level. The game doesn't drastically change beyond that level unless using house-rules or a later edition that adds significant abilities for higher level PCs.
With many campaigns ending while still at lower levels do the "balances" based on leveling make any sense? I prefer a humanocentric setting and I think the class limits for demihumans help achieve this by discouraging some players from taking demihuman characters. Even if they never reach the class limit, the idea of being limited may be enough to discourage interest. Most of us tend to dream big and imagine our paper hero growing powerful one day. I think race-as-class has a similar effect also limiting the number of non-human PCs in the game because they aren't special enough. It's a subtle effect, but subtle limits are usually best. A similar side-effect may help to keep the number of magic users lower than the number of fighting men and clerics. Magic users with a half-dozen or less hit points, no armor, a dagger and one spell can have limited appeal as a PC.
A nice feature of White Box is that higher level PCs are not so powerful as to be immune from even 1- hit die monsters, who in numbers can still pose a challenge. That tribe of orcs that have been a thorn in the PC's side since 1st level remains a force to contend with at 10th level and beyond. In many ways the lower power level of PCs means the White Box version of the game seems more "realistic" than many later games where PCs become supermen and at higher levels, virtually immune to the threats of lower level monsters.
The basics of leveling are to reward success in the game by making the player's alter ego a bit stronger, able to tackle problems with a higher probability of success or to take on greater challenges previously beyond the capabilities of the PC. Improvements in hit points and the ability to hit in combat, improved saving throws and increasing number of available and power of spells for casters is what gaining in level means in terms of mechanics. Growing out of the war game where playing pieces have an unchanging value, this concept of getting better at the things one does is a novel idea with a feel that brings "life" to the playing piece. People learn and get better with practice and the level mechanic allows this to happen in the game. There are other ways to mechanically simulate the learning process and many games have developed systems other than "leveling", but White Box's class and level system redefined the nature of war games even laying aside the fantastic elements and "magic".