First, you are legally required to maintain accurate records: all your invoices, bills, bank statements etc. relating to the business must be kept for 5 years. Failure to keep accurate records can lead to financial penalties from HMRC.
Just as importantly, you need to know how your business is doing financially. Is it making a profit, what your cash flow is like, who does the business owe money to and who owes you money?
Having up-to-date bookkeeping records allows you to see the health of your business at a glance!