首先,英语中有一个规定就是一般不会在句尾放介词（介词后面一定有名词）.所以在碰到从句的时候,特别是which或者that所指代的词在从句中视作为介词后面的名词出现的时候,就必须用which,并将介词前置.
which在这里是指代那个the city of Washington的,后面的从句的应该写成the village of Georgetwon was the market center for the city of Washington.
明白了吗?给你举个例子
His first job is in a big bank,from which he learnt a lot of information about financial management.
which是指代bank的,后面的从句是说he learnt a lot of information about financial management FROM the BANK.
所以不存在for which的用法,只是介词加which的用法.