The timer/counter "peripheral" section of a microprocessor is kinda the Swiss Army Knife for all kinds things related to, well, timing and counting.
mind-numbing hard to slog through the datasheet section without a specific goal, but recommended, if only so you can see what all they are capable of, and how many of what sizes and capabilities you are blessed with on the chip you are using.
Later you'll just keep coming across things that get way easier (and harder :-|) to do with timer/counters and you'll remember, maybe, to delve back in and sort the possible pesky problem of configuring the control registers and working out some constants to achieve your goal.
It can unburden the central process, if it was something you could have done in straight ahead code without the timer/counter, and for higher frequency applications make possible things you just could not do otherwise.
Interrupts play a role in this, so it is imperative that you come to this peripheral with a good hold on the hows and whys of interrupts.
The availability and feature of the timer/counter(s) is one of the central differentiating specifications of a microprocessor, right there with program memory, RAM, EEPROM and so forth.