A wrapper is a way of providing extended functionally with the use of inheritance, or simply providing an alternate API. The uvm_event_base class is doing both.
The SystemVerilog event construct give you syntax to trigger an event ->e and wait for an event**@e**. The uvm_event_base class provides methods to the exact same thing with the trigger and await methods. By using those methods it is also able to provide extra functionality like counting the number of processes waiting for the event to trigger.