Whenever a user records a time item in an AceProject timesheet, he/she must associate a Time Type to the item. From a reporting standpoint, both the project level and Home tab time reports allow you to filter by Time Type. Hence, your decision on how to use time types should take all this into account as well as the fact your time types will be identical from one project to the next.
Time Types could, for instance, be used to distinguish billable from non-billable time. Should you use Time Types for that purpose, bear in mind that it is not currently possible to create multiple billable and non-billable time types and then be able to capture time associated to all billable or all non-billable time types in a single built-in time report the application provides.
Time Types are also commonly used to specify and monitor vacation time taken by AceProject users (using a time type called “Vacation Time”, for instance).