A Bot is an (semi-)automated account for performing the repetitive tasks which would be extremely tedious if done manually (e.g. changing a capital letter in a link). As a Bot is designed for fast repetitive edits, it can do a lot damage in a very short time. This may be due to a small human error (e.g. adding a space too many) or for malicious intent. This is one of the main reasons the Bot Policy was put in place.