Expenses can be used for anything you want to use them for. They can be Reimbursable or not. If Reimbursable is un-checked, then when you create an invoice, that Expense will not show up. So if you want to track some expense, say for tax reasons, but don't want to invoice it, then you un-check Reimbursable. You can then run a Report on the non-reimbursable expenses at the end of the year.
Other than that, you can put anything you want in an Expense. You can enter one Expense for the $120 for the year or you can create an Expense for $10 and bill monthly. It does not have to be tied to anything. Some people even use expenses for billing time. Some people bill for one week and want to put it in as one entry instead, so they use the Expenses. Hope that makes sense?