Unless you intend to do the books yourself, it's a good idea. If it's a large business, count it as an essential, might even keep you out of jail.
I learned something new about business every time I met my accountant. She looked at the business in a completely different way from me.
Tax laws change every year, and it is the accountant's job to keep up with them. For that reason alone it is worth hiring an accountant. If your business is small enough you can keep the day to day books and hand over your ledger and receipts once a month.
No its not mandatory .. You don't need an accountant for your day to day business, a book keeper would suffice for that....But, it's important to know and keep up with changing laws and regulations, prepare financial statements at the end of your fiscal year and file taxes properly and efficiently. If you know how to do all of these things .. Then, no .. You don't even need an accountant.
I think this depends upon the complexity of your business. If you are just starting out, and it is a part time business that you are able to maintain the "Books" for, then I would think you would be capable of "Keeping up". There is a point to your business's "Growth" where you may have to reevaluate your "In-House" book Keeping to a point of hiring an external resource to do this for you. Then reevaluate once again if the business grows to such a state that you have your own "In-House" employees handling the books again.
My Wife does a great deal of "Book Keeping" for young and small businesses in her area, so I get to see these businesses at their various stages of growth.