You may have a need to keep multiple versions of an executable (like java) on your system. Perhaps most of your system will work with Java 8, but one application needs Java 7.
The alternatives program lets you switch from one version to another, quickly.
EDIT: For other reasons you may want to do this, see What is the difference between JAVA_HOME and update-alternatives? and Better way to add alternative using update-alternatives?
From man alternatives:
It is possible for several programs fulfilling the same or similar functions to be installed on a single system at the same time. For example, many systems have several text editors installed at once. This gives choice to the users of a system, allowing each to use a different editor, if desired, but makes it difficult for a program to make a good choice of editor to invoke if the user has not specified a particular preference.
The alternatives system aims to solve this problem. A generic name in the filesystem is shared by all files providing interchangeable functionality. The alternatives system and the system administrator together determine which actual file is referenced by this generic name.
