Using the same language is useful for avoiding duplicated code, personally I find these being duplicated often:

* Form validation/data sanitisation code

* Translation/internationalisation

* Data (de)serialisation

Also you can do fancy things like server-side rendering, although Google is capable of searching SPAs now. I don't know much about SEO.

The argument for less code sharing is that if you are working with a lot of teams and service-based architectures, there are some merits to duplicate your code and keep them redundant, to 1) enable hiring from more diverse backgrounds and 2) reduce communication/maintenance friction needed when you make changes to components would otherwise be shared.