Yes there are still client / server systems around and when you are running a small business on one it can be tempting to cut corners.
Put simply, client / server systems run software that handles jobs and request from external parties i.e. clients.
This is mostly seen in small businesses where they are running some bespoke (or old) software that two or more people need to share and/or access at the same time.
Client-Server systems are mainly about everyone using the same database to improve efficiency, communications and to reduce duplication.
It is a little "old-skool" now, with cloud based systems being the norm, but many industries are still running what appears to be ancient technologies on their local machines.
In this old-skool system the server should be a stand-alone machine that no-one touches or uses for any other reason.
But what usually happens is that the business tries to save money by having the server machine also be a client machine. It is easy to do and quite often seems like a good money saving strategy.
But this is far from being a good idea or a money saver.
In the diagram above, imagine that the Server and the PC are the one machine. It is easy to do in this multitasking world.
It isn't hard to realise that now you are asking your "server" to do 2 jobs. This means it has fewer resources to serve the needs of the other clients.
With a person using the server PC as a client it also exposes that machine to the foibles of human computer users.
There's a good chance that the human will overload the machine by trying to do too much, and when the machine frustrates them perform actions that further impair the serving of clients.
Because most viruses and malware are introduced by humans, a machine sitting on its own with no human attached is significantly less likely to acquire a virus, you can expect that your server and client combination will go down at some point in time.
This means all your clients are also offline.
If you'd just had the user on a separate machine, and that got a virus or whatever, the server would still be alive and the other clients still able to work.
The bottom line is that by having a human client working on the same hardware as the server you are going to impact the whole system's productivity and also expose it to significantly greater risk of total failure.
This was something that I point out to my clients on a regular basis. Some listen, some don't or can't.
The scenarios I described above aren't fictional. They've happened to my clients and the ones with dedicated stand-alone servers with nobody touching them invariably fare better when the inevitable problems occur.
So many times in computing short term financial gain (read: money saving) ends with significantly greater financial pain down the track.
For the cost of a single PC you could risk your entire business.
So don't be a client on a server.