What objective is ordered to the VMs for your planned project? What do they need to complete or what are they tasked?
Still not sure what exactly you're asking, but I think you're asking why? At work I manage around 400 "servers" across ~20 locations. Using VM's allows us to host multiple servers on a single piece of hardware, which is a huge cost savings in hardware, operational costs like environmental and electrical, and ease of management. We've got AD domain controllers, DHCP and DNS servers, file/print servers, application servers, database servers, and web servers.
At home, I use VM's 1-to learn, having a home lab with an entire functional domain has been a huge learning experience and it's fun. 2-To do what I want. If I want to learn exactly what a malicious piece of software does, I can detonate it in a VM locally without needing to worry about it destroying anything important. If I want to try a new linux distro, I can.