Google, through V8, Chromium, Chrome, Chrome OS, Android, and Dart, has a large say in the future of the web platform, browsers, apps, the future of Javascript, etc.
Time and time again, Google has used their substantial influence and market share to make unilateral decisions affecting the web platform [1][2][3][4][5][6]; the individual merits of each decision notwithstanding, Google can do just about anything and it will affect enough of the userbase that others must compensate or follow suit.
By offering an alternative to V8 monoculture, they can protect against Google using its influence to drive the Javascript platform in its own direction.
[1] SHA1 sunset: https://security.googleblog.com/2014/09/gradually-sunsetting...
[2] NPAPI discontinuation: https://blog.chromium.org/2014/11/the-final-countdown-for-np...
[3] Block Flash by default: https://groups.google.com/a/chromium.org/forum/#!searchin/ch...
[4] Chrome ignoring autocomplete=off: https://bugs.chromium.org/p/chromium/issues/detail?id=468153...
[5] VP9 available in WebRTC: https://developers.google.com/web/updates/2016/01/vp9-webrtc...
[6] H.264 in WebRTC still gated behind a flag: https://bugs.chromium.org/p/chromium/issues/detail?id=500605