As for why not Helvetica or Times New Roman, it's because they're boring. Using either of these typefaces is a non-choice. It's like not using any color in your design. If you do it right, your design can look timeless, transparent, minimal. If you do it wrong, you look lazy. Arial is a great choice for somebody like Google who wants to position their brand as if it's the very substance of the internet. Before the recent re-branding, they looked lazy (because they were). Now they look timeless. But if you want your brand to have a unique character, or you have interesting typographical problems to solve, you should look into other typefaces.