Why is colon resection still done for colon cancer in cases with liver metastasis?

Depends. Believe it or not, it can sometimes still be cured. It depends on the disease burden in the liver. After resection, they can do adjuvant therapy and follow up with hepatic resections if it is limited. Also for palliative reasons - if it is causing an obstruction, then the obstruction needs to be relieved to allow for simple gut function.
Variable. The good reasons are: 1) limited metastatic disease that is potentially curable with colon and liver surgery, 2) excessive tumor bleeding, 3) tumors that are obstructing the colon.In general, there is no major advantage to removing the colon otherwise, but sometimes it is done because the surgeon is not up on the latest literature . It is safe to try chemo first and operate only for symptoms..