▲你说得对，介词 for 有一个用法就是表目的。如：

What did you do that for? 你干吗这样做？

I have come for the dictionary. 我来拿词典的。

I’ve come for the policeman. 我是来找警察的。

I have come for the dictionary. 我来拿词典。

He’s come for yes or no. 他是来听答复的。

For the purposes of the survey we’ve split the town into four areas. 为了便于调查 我们把这个镇划成4个区。

▲但是，介词for可用来表示目的，它习惯上不能后接动名词来表目的，遇此情况可改用不定式，如：

他去那儿看他叔叔。

误：He went there for seeing his uncle.

正：He went there to see his uncle.

不过，对于这个已经名词化的动名词，它可以与for连用表目的：

He went there for swimming. 他去那儿游泳。(swimming 已名词化)

▲另外，for doing sth 虽然不用于表目的，但可用于表原因、用途等。如：

He has two reasons for doing this. 他这样做有两个原因。

The teacher thanked them for being so cooperative. 老师感谢他们的通力合作。

This tool is valuable for doing electrical repairs. 这工具对修理电器很有用。

He’ll have some good reason for being late, I’ll be bound! 我敢肯定，他一定会为迟到找个什么冠冕堂皇的理由。