In general I think there’s a desire to be able to access the browser extension as a tab. Yes, you can pop out a separate window, but it seems many people would like a workflow where the vault is available as a tab, specifically.
Notably, this capability is available in LastPass, so many recent migrators (such as myself) may be familiar and comfortable with this workflow. The web vault isn’t a 1:1 replacement because the browser extension has different timeout and PIN unlock capabilities.
Examples of user interest (I’m new and can only include two links in post):