すいません基本的な質問です申し訳ありません。
WM_COMMANDで、return　0;せず、break;している狙いは何なんでしょうか？
教えてください。

LPCTSTR str[] = { TEXT(Kitty on your lap) ,
TEXT(Tokyo mew mew) ,
TEXT(Magical nyan nyan TARUTO) ,
TEXT(Di Gi Charat) ,
TEXT(Nekoneko Zoo)
};

enum{ BID_DEL = 2 , BID_RESET };

LRESULT CALLBACK WndProc(HWND hwnd , UINT msg , WPARAM wp , LPARAM lp) {

static HWND list;
int i;

switch (msg) {
case WM_DESTROY:
PostQuitMessage(0);
return 0;

case WM_CREATE:
list = CreateWindow( TEXT(LISTBOX) , NULL ,
WS_CHILD | WS_VISIBLE | LBS_STANDARD ,
0 , 0 , 200 , 100 , hwnd , (HMENU)1 ,
((LPCREATESTRUCT)(lp))->hInstance , NULL
);

for (i = 0 ; i < 5 ; i++)
SendMessage(list , LB_ADDSTRING , 0 , (LPARAM)str[i]);

CreateWindow( TEXT(BUTTON) , TEXT(削除) ,
WS_CHILD | WS_VISIBLE | BS_PUSHBUTTON ,
200 , 0 , 100 , 30 , hwnd ,
(HMENU)BID_DEL ,
((LPCREATESTRUCT)(lp))->hInstance , NULL
);

CreateWindow( TEXT(BUTTON) , TEXT(リセット) ,
WS_CHILD | WS_VISIBLE | BS_PUSHBUTTON ,
200 , 30 , 100 , 30 , hwnd , (HMENU)BID_RESET ,
((LPCREATESTRUCT)(lp))->hInstance , NULL
);
return 0;

case WM_COMMAND:

switch(LOWORD(wp)) {
case BID_DEL:
SendMessage(list , LB_DELETESTRING ,
SendMessage(list , LB_GETCURSEL , 0 , 0) , 0);
break;

case BID_RESET:
SendMessage(list , LB_RESETCONTENT , 0 , 0);
break;
}
return 0;
}
return DefWindowProc(hwnd , msg , wp , lp);
}