In most cases, it can be boiled down to "clips are used to load magazines." If the magazine is intended to be detached from the firearm (ARs) then you don't absolutely need to use clips to load it. But if you've got a bunch of rounds already on clips, it will save time versus placing the rounds in the magazine one by one. Clips are more of a necessity when loading rifles that have an internal magazine, like an M1 Garand.

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