In essence, custom tabs are stripped down versions of Chrome that load much faster than the full-blown browser but maintain many of the browser’s advantages, like saved passwords, sign-in, autofill, and security settings.
Plus, they look nice. Custom tabs can take on the color scheme of the mother app for a consistent look.
Why this matters: Google+ is an ideal training ground for Google to more widely deploy its custom tabs feature. This can show developers and everyday Android users that it’s an improvement over sending links to Chrome in terms of speed and features. This update appears to be a server side switch, so be on the lookout for it if you’re running the latest version of Google+.