Does the software you write have quality? How do you attest that?
Software is usually written according to certain requested needs, be it bug reports, feature and enhancement tickets, or whatever. To have quality, the software must satisfy these needs wholly and precisely; that is, it should do what is expected of it.
Just as you would push a button to know what it does (given you do not have a manual), you have to test your code to know what it does or to attest what it should do. That's how you assure software quality.
During the course of a software development, it is usual to have many features that share some code base or library. You could, for example, change a piece of code to fix a bug and ...