A question about dedicated server files

Hi guys, simple question:

I set up my first dedicated server for gmod.

The hldsupdatetool is downloading sounds files, textures etc.

Is it necessary for a dedicated server? I mean, are they needed for making a server, or are they totally useless?

Yes. If a model does not exist on the server, the model existing on the client is pointless - it will only be a clientside model - which will have no physics and such.