leave off和leave for的用法区别

leave off和leave for的用法区别
leave off 和leave for 好象都有离开某地去某地的意思.eg.when did them leave off ShangHai I'm leaving for Beijing.I shall leave HongKong for BeiJing.特别leave for和leave to 等有什么不同呢,例如上面的能改写成I shall leave HongKong to BeiJing吗?