When working on a project, often different tasks need to be done at the same time, e.g., implement feature in files X, fix bug in file Y and correct typo in file Z.
This causes your project to contain modified files from different tasks.
Change Sets allow you to organize these files into groups of related changes.
Committing your changes in task-oriented portions makes life easier for you and your team when it comes to checking the log.
You can also use the Change Report to review your changes and organize them into Change Sets.