Why do many Win32 Strucs contain their size as a member

This is not strictly a programming question but more of a design question.

Working with some strucs from the WinAPI I noticed that many of them contain a field called dwSize that simpally stores their size e.g THREADENTRY32 or BLUETOOTH_DEVICE_SEARCH_PARAMS

typedef struct tagTHREADENTRY32 {
DWORD dwSize;
DWORD cntUsage;
DWORD th32ThreadID;
DWORD th32OwnerProcessID;
LONG  tpBasePri;
LONG  tpDeltaPri;
DWORD dwFlags;
} THREADENTRY32;

Thinking about this design i find it intriguing why can’t the code that using those structures use sizeof?

What is the actual thinking behind this?

>Solution :

This is to support future versions of that structure that might have new fields added to it.