In addition to the other possibilities mentioned, another common scam is to pay you too much, and ask you to send back the difference. Then the original payment gets reversed or turns out to be fraudulent, and they keep what you sent.

A lot of those "make $$$ working from home!" things are scams like this. They'll offer you, say, $1,000 for a small amount of work. On payday, they'll send you a $1,500 check, along with some excuse for why it's too high, and ask you to send them the extra $500. A couple of weeks later their check bounces, and you're screwed.

Another technique is to just ask you for help transferring money. They might send you $4,000, tell you to keep $500 to compensate for your time, and send $3,500 to their friend/partner/subsidiary outside the country. Then the $4,000 evaporates. This one is especially clever since the original scheme was probably some form of money laundering or other financial crime, so the victim will usually be reluctant to go to the authorities afterwards.

Don't know where GP is, but in USA you can forge a check with bank and account numbers. Of course the check must still be cashed, but perhaps a string of deposits and new checks on different accounts would make this take long enough to get actual cash...

The stereotypical 419 scam, however, has some "plausible" reason why the main transfer has to wait on a smaller transfer in the other direction.

Because, in general, banks are terrible.

Wait, you want me to type in my username and password into your computer, and then you can take any amount of money from my account, and all everyone needs to do is trust each other?

I think that once they get all your bank info they will do a withdraw from your account? That is the only thing that makes sense to me.

Usually, you have to wire the fees in advance of the 'payout'. Spend a couple thousand to receive your millions.

The same information that could be used to pay you could be used to extract money from your account, right?