How structure and unions applied in real world?

The better question is, where are they not used? Relatively few numbers exist on their own: loop indexes, counters, universal constants, local constants. Except for loop indexes, all these constants should be given descriptive names - the use of "magic numbers" in source code is rightfully discouraged.

Structures provide the relationships between or organization of data items. Every data file, for example, is organized into a data structure(s). (Otherwise, how can a program make use of the data?) In fact, the whole drive/storage medium is structured.

Another easy example is time. Any given point in time has century, year, day, day of week, hours, minutes, microseconds, etc., depending on your accuracy requirement.

Unions are trickier. A union changes the interpretation of a data item. It was an int, now it's a char, in the future it'll be an int again. One very handy use of a union is to create a union of many different pointer types. Being pointers, they all occupy the same size (a memory address), and all have the same value (though bounding constraints might cause issues), but by incrementing different members of the union one can smoothly parse a serialized data stream into... a structure that makes sense of the otherwise anonymous data.