The Last Event option reports on payments that are currently at a particular status. For example:
Payments that have an AUTHORISED payment status but have not yet reached the CAPTURED payment status
or:
Payments that have a CAPTURED status but have not yet reached the SETTLED status
If you do not use the Last Event option, the report may contain payments that have progressed to a further status. For example, if you generate a report for the CAPTURED status, without selecting the Last Event option, then the report may include payments that have subsequently been refunded.