histc counts the number of items that fall into bins whose edges you specify.
histc (or equivalent) are the fundamental routines behind creating histograms -- histograms require counting the number of items that fall into each bin and then using a bar plot on the result.
There are other uses for histc as well, such as counting the number of occurrences of items. For example if you had a vector of values representing apples, oranges, and grapefruits, then you could use histc to count how many apples, how many oranges, how many grapefruits.
As well as just counting, histc() can tell you the number of the bin that each item falls into, which can be useful in many different situations.
