With older desktops and laptops that can only support older browsers getting around the Internet can be difficult. HTML5 is prevalent and many common web apps require you to have access to it. In the case of Internet Explorer 6 and 7 and other common versions from yesteryear Google Chrome Frame provides some of these features easily and efficiently.
Many common web apps today make liberal use of HTML5. Google's own Google Drive uses it to facilitate real-time collaboration. Games use it to provide features that wouldn't otherwise be even remotely possible in a web browser. You get the idea. The point is for the browsers that need it to support HTML5 (or at least a small subset of HTML5's features) Google Chrome Frame provides compatible on par with contemporary versions of Chrome ? not every website benefits from GCF but many still do.
Framed the right way
Basically Google Chrome Frame is useful to users of browsers (and thus computers) from a 2004 through 2009 zeitgeist that many computers have moved beyond. It allows you to use certain features of websites (such as real-time collaboration) that are not otherwise supported on the browsers that Frame is meant for. In that respect treat this plugin as a quick-and-clean solution to a specific problem and use it if you need it.