The register is the practice's spine. Every act recorded in order, retrievable decades later, is both a professional requirement and the thing somebody will eventually ask about.
Identity evidence has to be retained, not merely checked. What was seen, when and by whom is the record that matters if a document is ever challenged, and remembering that a passport looked fine is not evidence.
Documents physically travel. Sent for legalisation, held at an embassy, returned by courier: knowing where a client's original is at any moment is the single most common question the office is asked.
Disbursements disappear. Legalisation fees, embassy charges and secure postage paid on a client's behalf are small individually and substantial across a year, and they are only recovered if recorded against the file.