Depending on the theme, function and target market the font will vary to fit in. Responsive typography is also something to consider allowing the text to be readable on several platforms opening up the possibilities for access from clients. Helvetica is formal font used more in a business manor.
Product branding is important to get the product noticed. Different colours and fonts are used sometimes in conjunction with a logo or tag line. This is designed inform potential clients at a distance of what they actually do.