Why use any 'raw' computer language over Excel or Number?
Excel and Numbers are easy to use and have a fairly intuitive set of operations which make them a good starting point for working with small-ish datasets performing a variety of common tasks. A reason you might want to invest in working with R, Python, or Matlab might be:
Working with large datasets
Running more complicated analyses (especially text analysis)
Producing customized data models, integrated analytics, or custom graphs and plots
Producing publication quality graphs and plots
Improve the reproducibility of your work
If you are interested in working with data to bolster your research efforts, consider these resources to help you start using these tools more effectively.