so i can think of two purposes but one of the purposes is still for the second one lol?
1. to make money
2. because the single albums/songs on their own aren't strong enough for a full fledged comeback and the group already has stuff planned
(i lied there's a secret third reason: artist/company discovers or creates other songs that would go well with the concept of the album they just put out, so they repackage. so... timing)
Number one and probably best executed example is Taeyeon's debut studio album, "My Voice". The lead single "Fine" is about a breakup and overall the album has a very authentic and ~Taeyeon-esque~ vibe. "My Voice" was released on February 28, 2017. The album itself, even down to the cover, fit the "later winter transitioning into spring vibes". On April 5, 2017, the deluxe edition (repackage basically) of "My Voice" was released with the lead single "Make Me Love You". The song itself has immaculate spring vibes and even by watching the music video or looking at the album, you can gather this. It's a song that fits perfectly with the rest of the album, just released at a more fitting time (April). To add to this, one of the other tracks that were added, her single "11:11", finally had its home on a physical release.
Releasing "Make Me Love You" on its own with "I Blame On You", "Curtain Call", and "11:11" in an EP would've made no sense. It certainly wouldn't have looked right tacking those songs on to what would be her next and darkest release (at that time), "Something New" in the summer of 2018. It may seem silly but I'm sure there are a lot of people behind it, including the artists looking at whether songs and concepts match sonically to make these decisions in order to make their releases feel more fluid and less choppy.
either way money is made so.