Fonts like Helvetica and Arial are commercial, meaning that one needs to purchase (rent actually) a WebFont license when providing that font.
However, both fonts are available on the very most systems and browsers. Instead of embedding the font into the website, one can simply say
font-family: Arial, Helvetica, sans-serif; in the CSS which makes sure the browser retrieves the font locally, on the user's hard drive. This is different from the first case, where the website provides the font.
In the latter case, is there still need for a license?