Poses are... poses or animations that are downloaded and not part of the actual game, and as such they require other mods to use. They can really add a lot if you're a Simlit writer or storyteller, as they allow your Sims to pose in ways that they otherwise would be unable to. Unless you take a lot of screenshots or write stories about your sim, they won't be much use to you as I *THINK* they just make your sim stand in that pose until you cancel the posing action.
I write (when inspiration strikes in a useful way), and I'm REALLY interested in learning how to use the pose player, teleporter, etc. (mods that are required for using poses) to improve my screenshots.
Basically what @SeaDragonSong said. I also write stories for my sims and use poses for screenshots. There are a plethora of poses, many of which are a lot more realistic, common, and appropriate for any given situation, compared to the default animations.
As for the gallery, it’s somewhat useful for close ups.
Like others said, I use poses both for storytelling and for general photosets! You can do some really fun things with them. Here's an example of a storytelling post with both poses and just regular gameplay captures - the three photos in front of the tree (images 1, 4, and 5) are poses, the two sitting on the couches and the last two are just gameplay. This set is 100% gameplay, and this set is mostly poses (aside from the one of them in the water, and even then I had to use MCCC to change them back into their outfits.) With gameplay, you're a lot more limited in the images you can take - I wouldn't be able to have all four of them jumping into the water, say, and there's only one major selfie image you can use.
So yeah! Short version, it's for storytelling and photosets and gives you a lot more freedom in what you can portray.
Poses are also great if you like to have family portraits hanging all over the house. I'm not the biggest storyteller player because of abandoning save files, but I like having the family portraits everywhere. It's cool to see generations grow through pictures.
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I write (when inspiration strikes in a useful way), and I'm REALLY interested in learning how to use the pose player, teleporter, etc. (mods that are required for using poses) to improve my screenshots.
Twists In Time And Space (Updated December 2nd 2018. New discord server!)
Bob Bobson (Updated August 12th 2019)
As for the gallery, it’s somewhat useful for close ups.
So yeah! Short version, it's for storytelling and photosets and gives you a lot more freedom in what you can portray.