Why Should I Use an Electronic Drug Register?

Because the government say so:

"The Drugs Inspectorate consider that computerised registers would be more useful than manual records ..."
Home Office Circular 48 / 2005 Controlled Drugs: Amendments To The Misuse Of Drugs Regulations 2001

"Computerised registers also facilitate maintenance of a running balance of stock ..."
Home Office Circular 48 / 2005 Controlled Drugs: Amendments To The Misuse Of Drugs Regulations 2001

"... the Government intends, subject to Parliamentary approval, to make inclusion of a running balance a mandatory requirement once computerised registers are in common use."
Home Office Circular 48 / 2005 Controlled Drugs: Amendments To The Misuse Of Drugs Regulations 2001