You have done plenty of work with student height. In reality, we also need to process other information of students such as their name, address, grades etc. As an example, lets think about student weight. Can we save height and weight data in a 2-D array? If you tried, you will realize it doesn't work - 2-D arrays still store only 1 piece of data. The way to locate that piece of data is two dimensional, not two pieces of information such as height and weight.
So, we have to create two arrays, one for height, one for weight. These two arrays are "parallel" because elements at the same index store the info regarding to the same student. How to make sure the data in two parallel arrays are "in sync"?