Mentioning people in Gmail—and having their email contact be immediately linked—is a feature that can save you and your email recipients a lot of time. Similar to mentioning a person on Facebook, this new Gmail feature allows your recipients to contact the tagged person with just one click.
It's not unusual to have more than one Gmail account. Maybe you use one for business and one for your personal correspondence. Perhaps you use one for a hobby, such as couponing, or for a side gig. Whatever the reason, Gmail makes it easy to move between accounts.
Do you belong to a beer league or social group? Do you frequently collaborate on projects with the same people at work? Is your family large and growing? Setting up custom email groups in Gmail can make it easier to communicate with these sets of people.