linux - why to use these weird nesting structure -

i'm trying study linux kernel , reading kernel code, can't understand structure use page structure shown below: mean,why use union nested in struct nested in union (the code simplified...)

struct page {     unsigned long flags;     struct address_space *mapping;      struct {         union {             pgoff_t index;               void *freelist;          };          union {             unsigned counters;              struct {                  union {                      atomic_t _mapcount;                     struct {                         unsigned inuse:16;                         unsigned objects:15;                         unsigned frozen:1;                     };                 };                 atomic_t _count;             };         };     }; } 

it used bring clarity code. easier read , understand if members grouped.

since not using 'sub-structures' in other data-structure, nested. else, declared separate , included members, below:

    struct list_based{                 pgoff_t index;                   void *freelist;      };      struct page {         unsigned long flags;         struct address_space *mapping;         struct list_based lpage;     };      struct sector {         unsigned long sub sect;         struct list_based lsect;     };