[c] Why do we need C Unions?

Unions are great. One clever use of unions I've seen is to use them when defining an event. For example, you might decide that an event is 32 bits.

Now, within that 32 bits, you might like to designate the first 8 bits as for an identifier of the sender of the event... Sometimes you deal with the event as a whole, sometimes you dissect it and compare it's components. unions give you the flexibility to do both.

union Event
{
  unsigned long eventCode;
  unsigned char eventParts[4];
};