The penis has a number of uses. Here is a short list of some of the more common ones.
The primary use of a penis is as a liquid waste disposal system. The vast majority of the time when someone uses their penis, it’s for urination. The flexible tube shape is often convenient for this purpose.
The penis is often the primary source of sexual pleasure for people who have them. Much of the penis can become sensitive to stimulation in certain situations. Not everyone who owns a penis will use it in this way.
Under certain circumstances, sperm from the testicles will be mixed with other fluids and pumped through the penis and out its end. Although this uses some of the same infrastructure as urination, this is a very different scenario. Not everyone who owns a penis will use it in this way.
Some people who own a penis enjoy using it with another person. On the other side, some people enjoy having a penis they do not own get used with them. Not everyone who owns a penis will use it in this way.