When to Nest a union inside a structure as a design choice

This might be a naive question , I have clear Idea about structures and unions ,and I have seen the use of nesting a union inside a structure in network programming and User Interface programming. But Since these codes are there from long back , I am not able to figure out what would be the design choice or advantage that leads to this decision. Why would I nest a union inside a structure and what would be the advantages , or is this a legacy carry over.The codes I have seen is mainly C.