Now you're touching on something called multicast. Typical network connections are unicast, and are "private" connections between two nodes. In this case between camera and server for example. If you make three separate connections to a camera, and each "client" connecting to the camera is pulling "stream0" which happens to be a 5Mbps h.264 stream, then there is going to be 15Mbps of data streaming out of the camera into the switch.
The concept of multicast is this: If you have many clients on the network all viewing exactly the same video as is usually the case with realtime data like audio/video, then it doesn't make a lot of sense to multiply that 5Mbps - especially if there are 10 clients all viewing the same video. You would quickly saturate the network when you have dozens or hundreds of cameras or more.
With multicast, the underlying network technology is IGMP or Internet Group Management Protocol. What IGMP allows the camera to do is broadcast it's availability to the network. All the IGMP-enabled routers/switches will "hear" this. At this point, there is no data streaming out of the camera.
When a multicast client wishes to "subscribe" to this video stream, they will attempt to make a connection to the multicast address. Multicast IP addresses are always somewhere in the range of 224.0.0.0 to 239.255.255.255. Now that there is a subscriber, the camera will start broadcasting the video stream, and the network will take care of getting that video stream out to the client.
As more clients subscribe, the switches make sure the stream gets to the client without duplicating the stream. So there is only ever one copy coming out of the camera. IGMP-enabled switches are also smart enough that if the camera is connected to a switch, and that switch is connected to another switch where 10 clients are connected, there will still only be one copy of that 5Mbps stream going between the two switches.
Whether or not you should use multicast depends a lot on the architecture of the VMS you're using, how you're using the system, and the network itself. For example, with Milestone, the only thing to "touch" the cameras is the Recording Server. So unless you have multiple recording servers or you have other applications talking to the cameras directly, there is little to no reason to multicast between the camera and server. But with Genetec, it is common for the client workstations to talk directly to the cameras, so it makes sense to multicast in that situation.
If you have multiple client workstations viewing the same cameras with Milestone and you're using XProtect Expert or XProtect Corporate, then you can multicast between the Recording Server and the clients. In that case, there is a single unicast stream from the camera to the Recording Server, and a single multicast stream from the Recording Server to multiple live clients.
Finally, it is very important that the network equipment supports multicast/igmp. If you're running "dumb" switches with no IGMP support, then the video stream(s) will effectively be "broadcasted" meaning the stream(s) will be sent out of all ports on the switch which can quickly kill network performance.