przemo's answer, imo, most closely hits the practical points. Here are some additional practical considerations:
Enterprise environments typically use separate partitions for at lease / , /home , /opt , /var , /boot , and additional filesystems (1 per application or application team) under /opt. This is primarily to avoid running the system out of space because of someone home directory getting massive (/home), logs going out of control (/var), or apps consuming tons of space (/opt , /opt// , etc), and /boot so that the other partitions can be built in LVM and also to ensure you can get something of a recovery shell should the primary system partition become corrupt for some reason.
For my own personal non-server uses, I just keep separate / , /boot , and /home partitions so that I can dual boot multiple Linux/UNIX OS's and use the same /home partition for them.
In the case of building a VM in Oracle Virtualbox, VMWare Player/Workstation/Fusion, etc for personal use, there really is no practical reason to have multiple partitions with different mount points on account of the nice folder sharing capabilities provided by at least VirtualBox and VMWare Player/Workstation/Fusion. The single exception might be /boot for the recovery shell reason given in my "enterprise environment" paragraph.