Twitter has started testing a new feature that allows users to sign up for an account using their existing Google account. This new Google account integration feature is first being rolled out as an A/B test on Twitter.com (web) The feature is entirely optional, and there are no direct benefits to the integration other than faster signup and the ability to use your existing account. It appears that this feature started rolling out over the weekend and should become available to more users in the coming weeks or months.
Source: https://www.bleepingcomputer.com/news/technology/twitter-will-soon-let-you-log-in-with-your-google-account/