Is it necessary to configure the windows server for 30-40 computers for a small organisation. our branch office is having the server and they managing all the server related issues but the hosts of our office is in workgroup configuration ?
It is generally recommended to switch from a workgroup to domain model after about 20 computers. The main reason and benefit is that with a domain controller on the network, you can centralize all of your user accounts and create other objects such as groups that you can easily reuse within the domain for protecting resources.
In addition, the server OS allows for the installation of other roles such as DNS and other networking services.
If you do not require any of these server related roles, then there is no reason to integrate a server on your network. I suspect that based on your description, you should consider moving away from a workgroup model.