In short: support for WebKit (to test Safari), out-of-process automation (not constrained by scope of in-page JavaScript, for example, to test multiple pages/frames/domains), support for non-JS language bindings. For a more comprehensive list, check out "Why Playwright?"[1].
[1] https://playwright.dev/#version=v1.4.1&path=docs%2Fwhy-playw...