Not only does this make you look more professional, this also helps you when you're juggling many projects even for the same client. Your client can simply tell you the job number of the project they're asking about and you can look at your page of job numbers and what the project's about in your records.
Tip: Don't start with the number "1" when assigning a job number for the project. And don't start with a rounded number like 500 or 1,000 either. Select a number that doesn't make it look like it's your first freelancing project.

