I think people say "sent in" (with no indirect object) when they are thinking of the act as one of submitting information to a system.
"Transmitting" the information requires having in mind a particular recipient who has been waiting for this specific information -- say, a functionary in the Government who you're having a problem with, or a person with a job to offer you.
"Submitting" the information involves having in mind an impersonal mail collection bin where large numbers of identical submissions are merely processed automatically -- entering a sweepstakes, for example, or subscribing to a magazine.
> "I'll send you my resume right away."
> "I sent in a box top (along with my name and address) for my secret decoder ring, but I never got one."
> "I sent her a love letter on February 14."
> "I sent in my tax returns on April 15."
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