Can someone explain prepayment meters to me

[BleurghUna]

Thought these were designed to help people budget for their fuel payments throghout the year, by paying the same amount into a meter every month. But I have read a lot of threads on here by people with meters saying they are worried about their winter fuel bills. Surely if they are prepaying for their fuel they won't get a bill? Or am I misunderstanding?
WE pay for our fuel by monthly direct debit, duel fuel, online, this si supposed to be the cheapest way, so why doesn't everyone do this, especially those who are struggling financially?