Each host within a vSAN cluster has a local memory read cache that is 0.4% of the hosts memory, up to 1GB. The read cache optimizes VDI I/O flows for example. Apart from the read-cache vSAN only uses data locality in stretched clusters to avoid high latency.
Whether data locality is a good or a bad thing has turned into a philosophical debate. Its true that data locality can prevent a lot of network traffic between nodes, because the data is physically located at the same node where the VM resides. However, in dynamic environments where VMs move to different hosts on a frequent basis, data locality in most cases requires a lot of data to be copied between nodes in order to maintain the physical VM-data relationship. The SDS/HCI vendors today that choose not to use data locality, advocate that the additional network latency is negligible.