IBM FlashSystem V9000 storage - should the os, mdf and ldf and tempdb be separated onto different volumes?
One reason for separate drives even with SAN is that a given disk or set of disks can only do so much I/O at once. Any of the things on the drive could saturate the I/O on the SAN by keeping OS, TempDB, logs, page file, and the user database on the same drive. Depending on the query and server configuration, TempDB, the page file, and the LDF could all be in contention for the I/O. By separating those, the I/O for those set of disks is exclusive to where they are allocated.

Another thing to consider is any one of those things could also use all the available space if sharing disk space and instead of one component shutting down, the whole server could be shut down.