To learn more about using Galaxy, start here: https://wiki.galaxyproject.org/Learn
If you are curious about a particular area of biology, a google might be the best place to start. Read some publications on the subject. See which tools are used by the researchers doing that sort of analysis and follow it back to that tool or similar tool in Galaxy and the help on that tool's page (author web sites, manuals), examples/tutorials in our wiki, and other help hosted by the community. You can also search prior Q&A or ask a new question here.
In the tutorial you just did (this one? https://usegalaxy.org/u/galaxyproject/p/galaxy-101-ngs-variant), variations were used because this is a common analysis done with Galaxy and it demonstrates several functions. Scientists are interested in transcriptome/genome variation because these changes describe the differences between individuals. If those individuals have different phenotypes (for example, one group has a disease and the other does not), the changes can be compared and potentially used to characterize the mechanism of the disease, to identify the best therapeutic, to develop an assay for future diagnosis or risk assessment in other populations, etc. But even that is a speculative, incomplete statement! I encourage you to read more about this. No short answer on a help list will give you the full "why bother" for these larger biological questions.
Hopefully this will get you started, Jen, Galaxy team