网友理解正确。省略介词与否，结果是：

Why don't you get a backpack to carry your books in? （定语）

Why don't you get a backpack to carry your books ? （目的状语）

According to Chinese tradition of the water splashing festival, the more water you pour over someone the more luck you give them, so we give each other a good few hours of hair soaking and reddening luck to remember each other by. （定语）

According to Chinese tradition of the water splashing festival, the more water you pour over someone the more luck you give them, so we give each other a good few hours of hair soaking and reddening luck to remember each other. （目的状语）