The MS range of grading exists because many business strike coins, while being uncirculated may suffer from defects.
For example, MS63 refers to a coin that may have minor bag marks or weak striking or some other minor flaw that was not due to being used in circulation.
MS70 refers to a perfect business strike with no flaws. Essentially, it's the best circulation strike you can get.
The Proof level coins have different degrees of flawlessness as well. Essentially, the higher the number, the more you pay for the coin.
Hope this helps,
Dean