Consider this data:
total part percent 765 47 6%
Clearly, there is some redundancy. Both
percent express the same thing.
With infinite precision, you could use either
percent at your pleasure. However, in the common case where the counts (
part) are integers and the percentage(s) are not, computers will store the integers generally much more nicely and compactly than nasty decimal things (floats or string representations).
Percentages also commonly get rounded off, in which case information is lost. In the above example, 6% of 765 could be anything from 43 to 49, and possibly even more depending on what precision is used for the calculation.
The moral of the story is that for data, you should always use counts, not percentages.