Why are they available? (rhetorical question)

Not sure why you call it a rhetorical question, unless you are assuming everyone knows the answer. They exist because they allowed perl4 and earlier (which didn't have real references) to build complex datastructures and pass non-scalars by reference (though IIRC globs could also do the latter). They are also handy (but not necessary) for symbol-table manipulation. They also do a great job of allowing you to shoot yourself in the foot, hence the prohibition of using them under use strict.