In his poem "Ode an die Freude", why did Schiller use the definite article? Why not just "an Freude"?
With the definite article "die", die Freude becomes personalised, concrete. You address die Freude as if it was a person.
Without "die" the object of admiration remains abstract, unconcrete.
It would feel awkward to sing a song of praise for something unconcrete and abstract. It is more natural to imagine the praised thing as something concrete and personal.
Ode an den Pudding
Concrete idea of flummery. The flummery is presented as something present, tangible, close.
Ode an Pudding
Abstract idea of flummery. The flummery is presented as something abstract, remote, intangible; rather the mere concept of flummery, not a flummery that could be eaten here and now.
Quite consequently you would rather say
Ode an Gott
*Ode an den Gott
as God usually is not seen as something tangible, rather as something remote: an idea, not an object.
(Commenters stated that Gott is used like a personal name here, and therefore there is no article, and that this is the stronger reason to not use the article than considerations of proximity and tangibility. See below in the comments.)
Ode is like a praise to something, and
Ode and die Freude just means "Praise to the joy". The article is I think present because "Ode an Freude" Sounds a bit off, and as you hopefully know, Schiller also was a famous writer, and nobody who knows a better expression would just use something that sounds a bit awful. But the Language of Goethe and Schiller doesn't exactly follow today's rules of grammar. In fact, at that time there were no defined rules of grammar, as we have today, and that's one reason why their works are often pretty hard to understand even for native germans.
And the question asked also needs to have a certain length, so you need to describe your problem/question a bit more in detail.