So far we might have run the tests through some browser or environment such as Node.js. How can we be sure our code works in multiple browsers for instance? That is where services such as browserling, testling, BrowserStack and Travis come in.
I won't delve into details. Suffice to say the services mentioned make it easier to leave testing to machines. You definitely don't want to have to check all browsers yourself. You are better off spending that time on something else.