Why wrap a struct with a union?

I saw a code snippet from a good answer for Is it possible to insert three numbers into 2 bytes variable?

For example, I want to store date which contain days, months, years.

• days -> 31, months -> 12, years -> 99.

I want to store 31, 12, 99 in one variable, and will use shift operators << and >> to manipulate it.

//Quoted: the C code from that answer
    union mydate_struct {
        struct {
            uint16_t day : 5;    // 0 - 31
            uint16_t month : 4;  // 0 - 12
            uint16_t year : 7;   // 0 - 127
        };
        uint16_t date_field;
    };

Now, my question is to why wrap the struct with a union? What are the special benefits besides memory related concern?
PS: I know some typical usage to make sure memory size with union.

Because if it is just to use struct, it seems more direct and simple to use:

   typedef struct {
        uint16_t day : 5;    // 0 - 31
        uint16_t month : 4;  // 0 - 12
        uint16_t year : 7;   // 0 - 127
   } mydate_struct;

Update1:

Some conclusion about benefits to wrap a union here:

• Can initailize the year, month and day simultaneously

The advantage of using the union is that give union my_datestruct u; you can write u.date_field = 0x3456; and initialize the year, month and day fields simultaneously. It is defined by the implementation what that does, and different implementations could define it differently. There's a modest chance that the year will be 0x56, the month 0x08, and the day 0x06 (aka 86-08-06 — century not clearly defined); there's also a modest chance that the year will be 0x1A, the month 0x02, and the day 0x1A (aka 26-02-26 — century still not clearly defined). People have forgotten Y2K already. ----comment of @Jonathan Leffler

• You can read/write the whole number at once.(----comment of @StenSoft)