The original version of GWT was mainly about solving compatibility issues between browsers (of which there were many). Modern GWT/J2CL is all about leveraging the mature Java development tools, especially code completion. GWT can be used with just the standard DOM APIs very effectively these days via the elemental2 library.
That makes sense, but I also recall it was about trying to use the same language on the server as the client. And it worked as well as the efforts to use JavaScript is the server. Which is not to say it can't work. There are a lot of traps there, though.
