The Reimbursable flag is present in the Expense Items screen to set the default value of an expense as reimbursable to the employees by the company. For example, if employees pay for a project-related expense out of pocket, they can be paid back by the company if the expense entry is flagged as 'Reimbursable' because checking this flag generates a liability that can be paid using the Bill Payments or Checks screen. This enables you to track the project-related costs more accurately. Check out these articles for details:
If you incur reimbursable expenses a lot, you should check this flag for all such expense items.