There are different reasons but few important are like
1: One reason for this recommendation is I/O performance: For SAP HANA, the I/O requirements for accessing the data volume are different from those for the log volume (see chapter “I/O Sizing”) so that, usually, separate file systems for data and log volume are needed to reach the I/O requirements for both volumes.
The log volume is accessed using a sequential I/O pattern while the data volume is accessed using a random pattern. Therefore, with separated volumes, better performance can be reached for the redo log writing.
2: The second reason is data safety: If the disks of one volume get damaged or data in one volume get corrupted then the other volume is not affected