Google has said in the past "we make our money from usage of the internet, and so anything that increases internet usage helps us". That was their stated motiv

Google has said in the past "we make our money from usage of the internet, and so anything that increases internet usage helps us". That was their stated motivation behind things like Chrome - increase the speed and utility of webapps so that more things could be online. It works in this case because they are improving security and therefore user trust in online services in general.. The cynic in me is sure that that's not the only reason, but it's part of the picture.