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It's just a way to give two names and use them (possibly) in every day life. It is definitely not something that started recently or for religious reasons.
Also you should bear in mind that the "middle name" thing is only English and American... In many European countries, for example, people have one, two... or (many) more forenames, not one or several middle names.

I believe the tradition started with the Catholics, who are either given saints' names at birth or take them at confirmation, or both. I am not entirely sure of how it all works, only that it is done by Catholics. Some people use both names, like women named Mary Margaret or Mary Jane. Others use just the second name, like the women in Marie Antoinette's family, who were all formally named Maria but went by the second half of their first name. Marie Antoinette was originally known as Maria Antonia in her own country, Austria. Her mother was Maria Theresa and her many sisters were named things like Maria Anna, Maria Elisabeth, Maria Carolina, Maria Josepha, etc. They obviously could not all go by Maria, so they used their second name instead. I believe that the Catholics choose patron saints (in the case of the Hapsburg family, they chose the Virgin Mary) to honor them and court their blessing; I am not positive about the why of the naming practise, but I do know that it is a tradition that the Catholics have followed for many hundreds of years. Other people probably liked the idea and took it from there with names like Anna May or Lily Rose or what have you, but I am confident in stating that originally all double first names included a saint name.

I'd imagine the use of double first names is just a way to elaborate on the name, to make it more unique and differentiate the person from others with the same name. There are a lot of people named John. So if you're called John Thomas, that will make you a bit more of an individual.