For Gmail I think it could still make sense for those who have a Keybase account/key, but haven't yet downloaded the app (this was me for a while).
Just as an example, I have a work contact who I am often exchanging passwords with. We use Proton Mail for these, and Gmail for everything else. Saying "I sent you X on proton" is a lot more cumbersome than "here's an encrypted message", and being able to decode it inline.
We could use Keybase's webapp, but it still requires going to an external site to encrypt/decrypt messages. So it's not a big enough draw to change our workflow yet.
I understand the argument that Keybase Chat has more built in benefits, but sometimes you just prefer email to live chat. It's also an easier sell to download a Chrome extension once, than download an app and run it 24/7.
Admittedly that's just my use-case. I like to think of Keybase as easily accessible PGP functionality, so that's where I'm most interested in seeing it grow.
I know Google's working on the problem I described with their end-to-end extension[1], but it's been years in development so who's to say when it'll be ready.
Regardless, I think the product is great. Looking forward to seeing what you guys put out in the future.