FatFS timing in usb_host_msc example code

The usb_host_msc example code for the TM4C129X dev kit implies that there is a tick counter requirement in order for FatFs to work.

In usb_host_msc.c the comments preceding the SysTickHandler() ISR say:

//*****************************************************************************
//
// This is the handler for this SysTick interrupt.  FatFs requires a
// timer tick every 10 ms for internal timing purposes.
//
//*****************************************************************************

The SysTickHandler() has a single line of code, which increments a tick counter:

void
SysTickHandler(void)
{
    //
    // Update our tick counter.
    //
    g_ui32SysTickCount++;
}

The only place g_ui32SysTickCount is referenced is in the GetTickms() function in usb_host_msc.c. But GetTickms() never gets called by any of the other code.

I looked at the FatFs online documentation, and cannot find any requirement for a tick count.

What is the purpose of the tick counter in this example code, and where can I find info regarding FatFs' need for a tick count?

Thanks,

Dave