To hide their identity. This is one of the major reasons why people ask others to verify an account using their name.
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I know.
However, the point is, those people that engaged with this kind of transaction (account verification through other person's identity) will just try to verify the account. If the account needs more documents, the one who ask/paid him to verify will just ask the "victim" to send/pass another document, in which he might be willing to do it for some money.
that's why KYC policies are not the ultimate solution and can even be harmful by incentivizing a parallel market for people's documents.