What is etc/alternatives used for?
There are some programs that are considered generic. Many programs can provide the required functionality.
For example, view is one such meta-program that can be provided by any tool capable of displaying text, such as more,less,vim,etc.
/etc/alternatives is a directory that is used in keeping track of the alternatives currently in use, by a tool called update-alternatives.
For more information, read the manual page by man update-alternatives in a terminal. Also visit Debian Alternatives System.