Why using Grid.Col error on next js app router ?

[zeffrypfs]

Like the title above, when using Grid.Col in Next JS App Router it's throw error :

Error: Cannot access Grid.Col on the server. You cannot dot into a client module from a server component. You can only pass the imported name through.

I'm know to fix this i should write "use client" at the top of file. But i'm curious why ?

[rtivital]

https://mantine.dev/guides/next/#compound-components-in-server-components

[zeffrypfs]

Thank you for the link 🙌