I think they’re trying to move to a browser / cloud model, presumably so that they can charge more for less.
So those are potential customers that JetBrains was/is losing. My brain is almost 20-years-wired for JetBrains products. I would have been much happier to be able to use a remote JetBrains IDE vs a remote VSCode.
There are also people for whom the existing JetBrains IDEs will always feel big slow and bloated. They have a lot of features packed in and come with a lot of lifestyle assumptions. I can see the value in JetBrains rolling out something that feels lighter-weight.