Do you want to drink something?
First:want to do是一个固定的用法.
Second:由于想得到对方肯定的回答,所以就用了something.
Do you want to have a drink?
Do you want to drink something? 我觉得是
Do you want something to drink?
补充：这个是一般用法，不需要什么理由。
Anything to drink？
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