it is mandatory to use OSDisk variable? if we did not use what will happen? second question if hard disk having multiple partitions which variable or logic needs to use?

The OSDisk variable is used to tell your "Apply Operating System Image" task sequence step which drive to install the OS onto. 

In that same task sequence step, there is a Destination option, where you can choose:

• Next available formatted partition
• Specific disk and partition
• Specific logical drive letter
• Logical drive letter stored in a variable

Thanks if hard disk having multiple partitions which variable or logic needs to use?

In the SCCM OSD task sequence, there will be a step called "Format and Partition Disk". In this step it will have the option to choose the Disk Number to use for the OSDisk.