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!

The issue names no files, tests, or entry points. Begin by reading the BCS serialization and signing documentation to explain why canonical serialization matters and how differing serialized bytes affect verification. Done means providing a clear, technically grounded answer to the relationship between canonical encoding and signatures.

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