Important for web apps or websites? There's a difference, a product's landing page will need SEO, definitely, but the app itself, ie app.example.com, likely does not need SEO. For example, an easy example is Figma. The landing page is built with HTML, CSS and JS, but the actual app itself is actually built with WASM [0], very similar to the canvas based implementation of Flutter apps.
https://www.figma.com/blog/webassembly-cut-figmas-load-time-...