The Event Viewer's PPU view allows you to visually check the timing at which various events (register read/writes, NMIs, IRQs, etc.) occur. This can be useful when trying to perform timing-critical mid-frame changes to the PPU, or to verify that PPU updates finish before vertical blank ends, etc.
The colors can be configured and it's also possible to define [breakpoints](/debugging/debugger.html#breakpoint-configuration) as marks to be shown on the Event Viewer. This is done by enabling a breakpoint's `Mark on Event Viewer` option. This allows the event viewer to be used to display virtually any special event that needs to be tracked.