In what cases we use unions in verification.?

[MichaelP]

To start, I would suggest reading a few previous posts on the forum, for example:

Structure and union

While a structure is a collection of different datatypes and a union is the same as a structure, but all elements share the same storage area, what is the difference between a packed structure and a union?

Additionally, I recommend reading some examples of union usage in System-Verilog available on the web.