“Mmmmm, my dear Watson.”
“What is it Holmes?”
“I deduce we have another”:

Today’s topic:
Collectibles
So, let’s just say you’re in a meeting with a bunch of colleagues making a game for a big company (don’t ask for details, it involved selling your soul to the devil somewhere down the line as well as some spoons and chimichangas. Quite a wild ride.)
You guys seem to have a problem.
“The game seems too short!” your colleague yells out, like the official exposition guy he is.
You think about it for a second. Then you have this AMAZING idea.
“Let’s add collectibles! Players will be able to get replayability from 100%ing the game!”
Well, that’s where I come in the room, looking like this:

Don’t ask.
Anyway, when you add Collectible Items in your game, you should think of more reasons for it’s existence than just to lengthen the play time. Replayability is just a bare minimum of reasons.
Collectible Items are all about opening up the world. Both in terms of explorability, but also in actual world building.
And don’t get me wrong, some games focus more on one than the other.
In a game like Mario or Sonic, the collectibles are more about exploring the world. It encourages you to look under every nook and cranny of the levels to find those Red Coins/Rings. There’s no reason why those floating Coins/Rings exist, there’s no backstories. They’re just excuses for the player to look at an alternative path and say “Hmmm… maybe there’s a Red Coin/Ring there somewhere…”

It gives players a reason to look at the level and see it as an actual environment to walk around in rather than just a linear obstacle course.
Then there are games like Alan Wake, where the collectibles don’t really add that much exploration to the levels (it does a bit, but the new paths aren’t really that special, like in Mario and Sonic where you’re only able to get there by doing skillful platforming and stuff) but instead it adds so many context to the story.
Due to the nature of this subject, I’m going to warn that there are some heavy spoilers on the game what I’m about to reveal here, so if you haven’t played the game yet, either just take my word for it and stop reading now, or go ahead and play the game and get back here.
Anyway, Alan Wake is about a writer… named Alan Wake, hope you can at least follow that much. He wrote a manuscript for a book, and something supernatural happens which made those manuscript come true. Cue plot-contrived amnesia and Alan Wake doesn’t remember ever writing them. Said manuscript pages are laying around the levels for you to collect and read, which generally gives you insights of past, present and future events, or even events that happened happened to other people in the world.

These are collectibles which add world building to the game. They not only have a reason for existing, they also explain why the protagonist is looking for them, and the story is made more powerful by having them.
Of course the best kinds of collectibles would be ones which have elements of both, but if you had to choose between one or the other, try to think of the strengths of the types of collectibles you’re putting in your game.
Simply having collectibles for the sake of having them won’t help your player like your game more just by having them.
It makes them play it longer, but simply having the player play your game longer doesn’t always mean it’s because they like it.