I believe the idea was new tabs open / append on right. So, just satisfy major usecase.
Say you come to hacker news and open top five links in a new tab, after visiting all 5 websites, you wouldn't want to close the homepage and the previous 4 websites. You usually want to close just the 5 websites you opened and not the hacker news page. So, user just have to right click on hackernews tab and choose "close tabs to the right".
I also want 'close tabs to the left' it would be useful, when you open 20 links and read through 15 of the links, you don't want those websites you visited to hog you memory, here "close tabs to the left" would be helpful.