With more people moving to cities, it provides enough labor force to the city, which is beneficial for cities to imrpove economy.

我分两步给网友解答该问题。

一、假定这个句子是正确的，那么，it 是指代另外的一件事情、事件，它不可能指代With more people moving to cities，就是说it 与它里面的任何成分都无关。这时，With more people moving to cities 是with 复合结构。该结构表达了一个含有逻辑主语的“主谓事件”，是后面句子的辅助成分，作“原因状语”。

二、如果没有上下文，而仅仅就是这个单句，那么，显然是用it 指代 with 复合结构，则判为错句。改正办法：

1. More and more people are moving to the city, providing enough labor force to the city, which is beneficial for cities to imrpove economy.

2. More and more people are moving to the city, which provides enough labor force to the city, which is beneficial for cities to imrpove economy.

3. More and more people are moving to the city, and it provides enough labor force to the city, which is beneficial for cities to imrpove economy.

我猜测，这个句子可能是“写作范文”中的句子。如果用it 指代前面的with 结构，是错误的，除非它另有所指。

此外，原句中，city 一会用复数，一会用单数，显然不合适。其实，可以用the city 表示“类指”。免得单、复数不一样。