The 13 bit Timer mode was designed into the 8051 to allow folks to easily "port" intel 8048 (the '51 precursor) programs to the 8051.

The 8048 had an 8 bit timer preceded by a "divide by 32" prescaler.

So the 8051 "emulates" this operation by using the low timer byte to act as the prescaler (incrementing the hi byte every 32 cycles) and the hi timer byte implementing the functionality of the old 8 bit 8048 timer.

Interestingly most of us didn't adopt this legacy mode prefering to implement the new timer modes.

regards,
p