Including the matter name on invoices is useful in most cases. A few settings must be tweaked in LeanLaw and QuickBooks to make this happen. The end result will be like this (high-lighted with the orange circle):
To enable the matter showing up like this on the invoice, first go to "Custom Form Styles" in QuickBooks. Then pick "Header" and "Custom Fields". Under custom fields you add "Matter" (or "Case" or "Project" -- whatever you want to show up for the client).
Once this is done, go to LeanLaw and head to Settings and go to the Invoice Presentation section. You should now have your custom field available to select under the dropdown under "Matter field on invoices".
Pick your custom field and click save. Now your invoices will have the matter name included.