A very very simple answer to your question. Please don't laugh.
in reply to why use a hash instead of an array
If you have 5 sons, would you like to refer each one of them with name or with indices ?
You must have understood that internally hashes are just like arrays- somehow named arrays - whose values are referenced using keys.
Arrays are better when a similar type of operation is performed on entire list.
Hashes are better when you would like to use individual members of a group, which are always better to refer via a name which you can easily remember during coding and not via an index.
Just my 2 cents on that.