The patcher is working in a non-destructive manner. If something goes wrong, your previous version of the files isn't destroyed. Otherwise you might end up with a corrupted installation folder and you'd have to redownload everything, not just the patch.
So basically you've got your current game client, the update files and the patched update files at least for a moment.
The patcher just needs that additional space as "working space". It is annoying, yes, but it's not really something they can avoid easily.
What @Smaxx said, plus the space needed for uncompressing downloaded data.
The minimum system requirements clearly state that at least 60GB of free HDD space is required for ESO. This number may have been updated recently, because I remember it being 50 GB. But as things get added to the game, so grows the client and its updates.