A simple way to understand why "some" is used is that it fills in the quantity part of the sentence.
It is too exact to say "I have seen six good films recently" as you sound too scientific and cold, or as if you are about to list them all.
To say "I have seen good films recently" sounds too vague, and is missing a vital part of the sentence.
I have seen some/a lot of/ a few good films recently, gives the quantity without sounding like you sit counting them, which makes you sound a bit of an otaku.
