The chain only assigns baking/endorsing rights per roll. Even if you are missing 0.00001 XTZ for another roll, it will not recognize that extra balance as a roll until the balance will make up another roll. Therefore, it's recommended to stake your excess balances to another baker.
Staking balance is just the sum of all the XTZ that will be used to count towards rolls. This includes your immediate balance in your baker address and all your delegators to your address. For a personal baker, this staking balance is likely the total of your own balance. For a public baker that takes in delegators, staking balance is useful to know how many rolls will be allotted for rights(more rolls = more rewards). This can be useful to estimate how much bonds you'll need from cycle to cycle. With the high number of delegators, it's not easy to know how many rolls you will be allotted for baking from cycle to cycle. This is where staking balance helps to quickly give you that information.
When people are referring to staking balance, they are likely specifically talking from this RPC route.