How can I close tabs to the left in Google Chrome?

Firstly why is this

I've always assumed this because it is useful to clean up after using middle-button to open lots of Google results. I find it useful. Since new tabs are to the right (and not to the left) of the current tab, this makes sense.

how do I add a button to close tabs to the left.

I don't know. However you can select a group of tabs using click and shift-click and then use Ctrl+W.

Use this extension: Close Tabs to Left

Too many tabs opened? Chrome allows to close all tabs to the right, close all tabs but current. But what about all tabs to the left? Tabs to the left are often was open earlier. This extension will help you to close already unnecessary tabs. Just click on extension's icon to close 'em.

You can select tabs that you need by press [Ctrl + click] on tabs which you want to leave on a pane. And after drag it to left part of pane. And use "close tabs to the right".

It's not a solution for your problem but not bad walkaround.