A full download of the game normally happens when corrupted data is found on your hard drive during the update so the system downloads everything again to try and overwrite the corruption.
PCs can get away with a "scan and repair" so just need to download the relevant information again but consoles can not do this so they download everything.
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Thing is, how do we know the patch didn't corrupt my Destiny 2 game files on initial download attempt? Because the game didn't do or had any problems before this patch.