Unlike opening bids in a suit, notrump opening bids have clearly marked upper and lower limits in strength and distribution.
Notrump openers are confined to three distributions, 4-3-3-3, 4-4-3-2, and 5-3-3-2. Of all the distributions, these three combined occur more often than all the rest put together, so it's important that you handle them accurately.
If you open one of a suit and rebid notrump, it's very easy for your partner to guide the bidding to the correct contract with natural forcing bids. But when you open 1NT or higher, you don't have the fine tools to properly direct the auction, simply from lack of space. Limiting strength and distribution helps solve the problem.
Starting at the top, here are the ranges for notrump openers and rebids:
In the last case, a rebid of 1NT does not necessarily promise balanced distribution. It simply denies the ability to go past 1NT safely. You would bid the same way with either of these:
J
K84
AQ97
K8652
Q85
Q82
J96
AKJ8