Delivery reports are a convenience feature that lets the sender know if the message they sent has been received (not read) by the recipient's device (for this, it has to be online and have sufficient storage space, though modern phones usually have so much storage the latter is no problem at all).
Every single phone I ever had, from early Nokias in the 00s to Androids and iPhones, had it disabled by default. While feature phones often delivered these reports with a pop-up and sometimes notification sound, which some people could deem annoying, this trend continues even with smartphones, which typically display it merely as an indicator in the chats list of your messaging application.
So, is there an actual reason why it's turned off by default everywhere? The feature has to be enabled on the sender's device to receive these and the recipient has no way of opting out of this, so it's not a privacy thing by any means.
UPD: Apparently, carriers in some countries charge customers for receiving delivery reports as if they were sent messages. I've never realized this - reports always were absolutely free where I live. Thank you for your responses!
That is still better than nothing :)
I'm not sure how exactly it works in Android, but I can see internal storage being freed after I move freshly installed, never launched (so, cache size should be zero) apps to SD, so I assume it does something with apps themselves as well.
--
I think I get what you mean now - to make an app movable, the developer has to set a value in AndroidManifest.xml, then the OS itself will offer an option to move that app elsewhere in app settings.