Parts and labour on separate lines
Parts come out of stock at a cost you paid; labour is time at a rate you set. Billing them as one figure hides both margins and invites the customer to compare your total against a workshop that itemises. Separate lines also mean the parts side reduces stock correctly, which is the only way the workshop's inventory stays true.
Service history per vehicle
The vehicle is the thing that returns, not the customer — sometimes with a different owner. Recording the registration number against every job means you can open a vehicle and see what was done, what was replaced and when. That history is what turns a routine service into a considered recommendation, and it settles the question of whether a part is still under your warranty.
Job cards and estimates
Work is agreed before it starts, often on a phone call, and the argument at collection is always about what was authorised. An estimate recorded and converted into the final bill — with anything added during the job visible as an addition — removes that. The customer sees what was agreed and what changed, which is a far shorter conversation than reconstructing it from memory.
Fleet and company accounts
A transporter, a taxi operator or a company with vehicles settles monthly across several registrations. Recording the vehicle against each job means the statement can be read vehicle by vehicle, which is what their accounts department checks before paying. Without that, a monthly bill covering fifteen vehicles is a number nobody can verify.