Regular Up To Date Financial Information
There are two main reasons why business owners want monthly/quarterly management accounts:
Your Own Benefit
You want to know in detail how your business is doing financially.
As your business grows, it's not always easy to keep track in your head of who owes you how much, what your turnover is, and how profitable your business is.
As well as simply keeping track of these things to help the general running of your business, it is particularly important to fully understand the state of affairs before making any major business decisions.
Third Party Credit
The bank/another third party wants them before offering credit.
It is a common part of a loan/overdraft agreement from your bank for them to require regular up to date financial information. They often also require these to be prepared by a suitably qualified accountant.
If you are a new business with no trading history some suppliers may request similar information before deciding what (if any) credit terms to give to your business.