FWIW this is also how crypto thieves can access your main account. I have had small random deposits made into my account. I was instructed by the brokerage to absolutely not accept them.I'll try to offer some suggestions;
Small deposits are often done before depositing a huge amount to check if the adress is correct so one doesn't deposit into the wrong account, or, it could be to trace it to a crypto exchange as a crypto exchange asks for KYC.
KYC means Know Your Customer which means one has to give their personal data to be able to use it.
Sometimes it's even possible to track a notification tied to a deposit which would mean they may be able to trace it that way.
These are just a few ideas as to why a small deposit has been made.
Ofc idk the actual reason so it's just all my opinion