Another reason, with small patches, especially those that actually do a binary patch instead of replacing affected files, you must apply every single patch that affects that file, and you must apply them in the correct order.

Back in the day when binary patching was the norm, this was a common pain point. Good patches at least did a minimal checksum check to make sure they had the correct starting point. Bad ones made no backups and just went to town, and failures meant reinstalling and trying all over again.