In an experiment to determine the effect of exercise on pulse rate student checks his pulse rate and after exercising for several minutes the purpose of checking his pulse rate before exercising is that

The purpose of this is to indicate the changes or effects in pulse rate bought by exercise.

We can also call this the control set up, it is where all the setting is same except without the variable that is going to be tested. So in this case, that variable is exercise.

So it is supposed that both set up has same settings such as the amount of time measuring the pulse, or the position of arm or neck etc.