The wayland protocol, as X, uses timestamps to match up certain
requests with input events.  The problem is that sometimes we need to
send out an event that doesn't have a corresponding timestamped input
event.  For example, the pointer focus surface goes away and new
surface needs to receive a pointer enter event.  These events are
normally timestamped with the evdev event timestamp, but in this case,
we don't have a evdev timestamp.  So we have to go to gettimeofday (or
clock_gettime()) and then we don't know if it's coming from the same
time source etc.

However for all these cases we don't need a real time timestamp, we
just need a serial number that encodes the order of events inside the
server.  So we introduce a serial number mechanism that we can use to
order events.  We still need real-time timestamps for actual input
device events (motion, buttons, keys, touch), to be able to reason
about double-click speed and movement speed so events that correspond to user input carry both a serial number and a timestamp.

The serial number also give us a mechanism to key together events that
are "logically the same" such as a unicode event and a keycode event,
or a motion event and a relative event from a raw device.