The money is available in the event of an emergency. You can draw it out anytime, it may cause a problem with your visa down the line, but if that happens, you can always use an agent.
A few years ago, to qualify there were people overstating their income, and swearing such to get "income letters" from their consulates. Immigration tried unsuccessfully to get the consulates to actually certify the incomes, and the consulates largely refused, leaving Thai Immigration to have to certify incomes on their own.
Legally, you now have to either show actual income, or maintain a balance in a Thai bank account. If you have the minimum required income, you do not need to maintain the balance.
The 800/400 is actually an easy way for people to maintain their visa without having to show income. It is an option provided by immigration to HELP people qualify that otherwise would not.
You do not have to deposit money with immigration, and immigration does not control your account.