The issue is that non-unique encodings are an attack vector, because parsers may in practice behave differently for noncanonical (or nominally invalid) encodings.
For example, you have an envelope format that goes: length prefix in LEB128, message, signature. One party controls the length prefix and signature, a different party controls the message. The message-writing party carefully crafts the message so that, in isolation, it appears innocuous, but when wrapped in the envelope, the first few bytes of the message look like continuation of the length prefix. Best case is the receiving party safely fails to parse the message, worst case is the receiving party successfully parses the message, verifies its digital signature, and interprets it differently than the signer did.
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The issue is that non-unique encodings are an attack vector, because parsers may in practice behave differently for noncanonical (or nominally invalid) encodings.
For example, you have an envelope format that goes: length prefix in LEB128, message, signature. One party controls the length prefix and signature, a different party controls the message. The message-writing party carefully crafts the message so that, in isolation, it appears innocuous, but when wrapped in the envelope, the first few bytes of the message look like continuation of the length prefix. Best case is the receiving party safely fails to parse the message, worst case is the receiving party successfully parses the message, verifies its digital signature, and interprets it differently than the signer did.