Benefit of dedicated log device (SLOG ZIL) in case of single device type ?

Hello,

What is the benefit of a separate log device on ZFS pool, in case the log device LUN is the same type (e.g. SAN device) as the regular pool device LUNs  ?

I can easily understand the benefit if SLOG devices are SSD or faster LUN, but if ZFS can write SLOG and regular pool devices at the same speed, what will SLOG improve ?

Is there any risk that ZFS will always counterproductively double-write data (on ZIL, then on regular pool) ? (on the erroneous presumption that flushing data to the SLOG is faster than flushing it to the regular pool)