If you receive cash from clients then this is really important
If clients pay you in cash then you need to have supporting evidence to show HMRC in the event of a tax enquiry, even if you bank all the cash received.
A booking diary or excel sheet is a useful way to support the cash income, the date, name or initials , the length of the appointment and the fee charged. Ideally a reconciliation of the cash received less cash taken out to agree to cash banked.
Remember if you have a tax enquiry HMRC will be looking to see if they can find evidence that takings were not recorded, especially in cash , so a record of some sort is essential.