Comment by cecilpl2

I feel like this has something to do with whether the identity of the thing is central to the story or incidental.

"On my last vacation I went to the beach to work on my tan." vs "on my last vacation I went to a beach to work on my tan". "The beach" feels like I went to a specific beach on purpose (or that there was only one), while "a beach" feels like I just picked one at random.

"On my way here I stopped at the store to get an apple" vs "On my way here I stopped at a store to get an apple". Similarly, "the store" implies intentionality/singularity, while "a store" implies I happened upon it.