Hello nnimicyy,
vSAN is a solution to share local-storage devices as a pooled resource between cluster members. Note that merely enabling this won't really do anything - you need (certified) controller(s) and SSD + SSD/HDD available to create Disk-Groups. HA and DRS can of course be enabled on a vSAN cluster for availability and compute resource load-balancing.
"When is recommended to setup vSAN?"
There are numerous use-cases for vSAN with examples such as workloads geographically-isolated from current infrastructure where SAN is not an option. Typically our customers choose to set up some clusters prior to or during their next storage-refresh.
Bob