Question

When do I use class boundaries?

Answer 1

If you have too many different values.

Example:
Say you measure the height of 2000 adult men. And you measure to the nearest millimeter. You will have 2000 values, most of them different.

Now if you want to give an impression of the height distribution in your population, you will have to group these measurements in classes, say 50 mm classes (under 1.50m, 1.50-<1.55m, 1.55-<.160m, etc.)

There are your class boundaries.

Everyone from 1.500 to 1.549 will be in a class, everybody from 1.550 to 1.599 will be in the next class, etc.

Now you may have sizeable class-numbers, that will allow you to make graphs like histograms, etc.