Usability is important for the whole purpose of using and human-made object. The ease of use with our without learnability is what we consider as a usable product. Before we take an example of the software product let us just think of some real-world problem.
Imagine you bought a bottle of new soft drink and with your prior conscious brain, you will look at the possible ways to open the bottle. One way is to rotate and open, other to pull up and maybe last option is to see if there is any lid to open. If all the above options fail, you get a little frustrated as you are not able to figure out. This is because the design of the bottle lid is not made in a usable way. After all attempts of typical usage types, if you are not able to open it and finally you discover that the opening is on the side and the cap is just a fancy thing, your temper will be on top.
As a user, you tend to learn things only if the product is intuitive to drive you towards learning instead of leaving one in chaos.
Similarly, when you use a software product, you see a login screen to enter email and password but no button to proceed, how will you log in? Here the usability issue is user is not able to login because of a fundamental bug. Like this there will be many minor and major bugs/issues in software which will be audited by usability experts keeping the users in focus.