Do I lose anything, and what do I lose, if I delete my ~/Library/Containers folder? (MacOS 10.14.6 Mojave)

An app could possibly store something other than preferences in their sandbox (i.e. container). What you lose would be dependent on the app that created the sandbox. As Kurt Lang mentioned, they mostly contain preferences or other configuration data for the app. However, I don't think anything prevents an app from storing data there.

Also, as was first mentioned, if you are looking to recover storage space, you are looking at the wrong place. The app container contains hard links to data elsewhere on the computer. When you look at the space used, those hard links will report the space used by the linked file, not what it actually uses (virtually nothing).

Motti.Sh wrote:

my Mac (suffering from relatively small SSD ...

If the real question is how do I free up storage space on my mac—

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How to free up storage space on your Mac - Apple Support

https://support.apple.com/en-us/HT206996

User tip: "Other and What Can I Do About It ?"

https://discussions.apple.com/docs/DOC-5142

Try something like OmniDiskSweeper for a GUI to get a good look at itemized file size and location:

OmniDiskSweeper http://www.omnigroup.com/more

others for example—

Disk Inventory X: http://www.derlien.com/ (takes forever to load up.)

Grand Perspective: http://grandperspectiv.sourceforge.net/

Or alternatively from the Terminal.app:

File size, and finding missing GB— list the items in the home folder with the sizes, including invisible items.

sudo du -h -d 1 ~/

File size, and finding missing GB —will list the items in root with the sizes, including invisible items.

sudo du -h -d 1 /

If you are trying to free up space on your mac do not delete items from your System folder or any Library folders, these folders contain items that are essential to the smooth running of your mac and the apps you have.

If you are looking to free up space copy items from within your Users/<Home> folder to an external drive. Things you do not need on a daily basis, movies images, music take up a lot of space, after copying them over and verifying they have copied over successfully then delete them from your mac.

What @Eau Rouge suggested is absolutely right. You should not remove the libraries and other system folders from your system.

If you do not pay heed to this, you may end up compromising the functioning of your system.

So ensure that you do not mess up your system.

You can buy an external drive and transfer the content that you don't need on a regular basis, to the external drive.

And then delete it from your mac.

That way you would be able to free up space on your mac for your endeavours.

Regards.

Despite other support articles claiming "Containers" will usually get cleared on next restart…

I don't know where you're reading these (or possibly even Apple's own support articles), but they are wrong. This folder is never automatically emptied out.

Lots of apps, both those installed by the OS and third party, put data for themselves both there and in the Group Containers folders.

Many of the Apple items are defaults and will be rebuilt if you remove them. But basically, they're another form of preferences, though a bit more complex than just a .plist file so more data can be saved.

I am greatly unsatisfied to find the SAME very photo assets in my Photos Library (~/Pictures/Photos Library/Masters) and within the 14GB com.apple.cloudphotosd container (/Users/motti/Library/Containers/com.apple.cloudphotosd/Data/Library/Application Support/com.apple.cloudphotosd/services/com.apple.photo.icloud.sharedstreams).

I already explained that. Please read about hard links.

There are not "copies" of those files. They all point to the same storage on disk.

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