Not everyone uses VMs to isolate stuff. VMs have many use cases and imagine for example you as an application/server/etc. developer want to quickly test whether your software works, reproduce a bug, etc. Being able to easily share data from your development system means quicker testing on operating systems that support this.

Of course that’s only one way to use it, but also think about the fact that most cloud providers mount storage in one way or another.

Also provisioning test environments might be easier. For example if I am a web dev using vagrant to have the server environment locally and therefor find any edge cases that are maybe unexpectedly operating system dependent using such a setup is easier than having to play with network in an operating system I might not be familiar with.

Vagrant is also used by the research community to quickly try something. Another environment where shared folders might be worthwhile.

I thought the entire point of vagrant was to deploy *isolated* vm instances