The Relationship Between Canonical and Signature in BCS

I am really confused about Why must the serialized object for signing be canonical.
The process of signing involves the sender's object being serialized into a byte array, which is then signed. The data transmitted is just byte data, and when the receiver verifies the signature, they simply retrieve the same byte data and perform signature verification. It seems like there is no connection to canonical at all during this process.
So, what is the actual impact of canonical on signature verification ?
I look forward to your response. Thank you!

Start by reviewing the BCS concepts described in the issue, especially canonical serialization and signature verification. Clarify why canonical encoding matters and how it affects the bytes that are signed and verified; done means the explanation directly resolves the author's question.

Written by the indexing model from the issue text.