I've got a question.
All TheSims 3 Store content sets have one small package file that is the set file, it has no other content inside it but it just seems to be a title, "combine" the whole set into one. I have inspected the sims3pack files of Store content sets with CUSTARD and package files converted from them with S3PE but I do not find any information on what do these set files do when installed into the game as package files. Are they used for some kind of Collections set or they just take space and give back nothing?

Do you find that small package file within all of the TS3 Store sets, or just some? If some, I'd suggest checking to see if those are the sets that they included in with Collections (within the game we can select Collections, which includes some of the store sets). Maybe that small file is related but I don't know how to find out for certain beyond comparing.

Every TS3 Store set has a set file, even the small free Valentine's day sets that have like 2 actual objects.

I think it serves two functions: telling you in the Launcher that you have the whole set and as some sort of collection file in Buy Mode. You can access these collections from the little golden square in the top right corner of the room category section.

They are rather useless otherwise and when I convert a set to package I just delete it. Or rather delete it in any case because it seems to increase loading times when installed as a package. The game treats it as an object but it's mostly garbage data and as such causes the engine to become confused.

Good to know. Thank you!