In general you create links when you want to have the same content reachable under different names.
Since some applications are looking for
add and other for
list-add, it is easier to provide both icons, then to change the software to only look for one of the names.
Another reason is that designers might want to keep the set of icons small, and thus provide related topics with the same icons.
Links in general are generated with the
ln -s target.svg linkname.svg creates a symbolik link between the two files.
You can read the documentation of ln by running
man ln or a shorter version