Recently, my grandpa picked me up at the airport when I was visiting family in Cleveland. After some brief conversation about the flight and North Carolina weather, he quickly asked me, "Can you help me get Google Chrome on my computer?" I was taken aback. My grandpa is definitely tech savvy for his age (he texts, emails, uses Excel, does research on the web, and more), but he was also content using a 10 year-old computer and had never previously brought up a browser to me.
I told him I would help out and agreed that Chrome was a great choice for him, but I wanted to know why he all of a sudden cared what browser he used and how he chose Chrome. The answer was simple: Gmail.
My grandparents had recently upgraded to AT&T U-Verse and changed to Gmail in the process, because their old email addresses had been linked to their previous Internet Service Provider. Gmail would be easy to use, is the fastest growing email service, and they wouldn't have to worry about space. It was an easy recommendation and both the installation guy and I pushed it to them. Similar transitions have been happening around the world since Gmail was released in 2004, building the service to more than 400 million active users as of June, 2012.
Gmail originally just seemed like a novel email concept, allowing people to have unlimited space and an intuitive web-based email client, and something Google was doing to make the web a better place for people (especially us nerds). Then, it became an important part of Google's ad business as they (controversially) started to serve ads into your inbox. Now, second to Google search, it is likely the main way people interact with the Google brand and its services. So, in their quest to dominate the web experience and your web existence, Google is using Gmail as a beachhead to get users to use their other web tools.
Trying send a big attachment? Load it up through Google Drive. Want the best and fastest email experience? Upgrade your browser to Chrome. Curious about the contact you're emailing with? Check them out on Google Plus (well, maybe that one hasn't worked yet...).
It's a smart model and seems to be working great (it's a primary reason I'll use Drive to store big files). Google chose a specific web services niche, email, executed extremely well and has now used that strong position to push farther into web services. It lets them gather your data, target advertisers ads, and continue to make gobs of money. And it lets my grandpa have a better web experience.