If you run your own business, no matter what size, bookkeeping is a legal requirement by H M Revenue and Customs.
You or your accountant will use these records to create a profit and loss account - which shows the sales income you've received and the expenses you've paid, and what profit/ loss you've actually made. Your tax liability will be based on this.
All businesses are different and there are many specific types of detailed records that may need to be kept. Some examples of records you should keep include:
If you do not keep accurate and complete records you may end up paying more tax than is due because of lack of evidence of tax deductible expenditure or/and inaccurate sales records causing H M Revenue and Customs to assess your expected sales .
If you pay an accountant to prepare your accounts they will charge you based on how long it will take them. If your records are more accurate this will reduce the time taken and therefore reduce the amount they charge.
The above reasons are sufficient to ensure you keep good books and records but the most important reason is to ensure you have control over your business and that you can assess its profitability and the cash flow situation therefore ensuring you are aware of any potential problems as soon as possible and can make business decisions with all available information at hand.