nagapie paying people early can make people worse off, if it brings the payment forward into a different 'assessment period'.
Eg, when I started a job a few years ago, I was due to receive my last benefits payment on 21st of the month, relating to an assessment period from the 15th of the previous month to the 14th of that month.
I'd started work on the 10th of the month, so should have been due UC for 15th of previous month to 9th of the month, so about three and a bit weeks money.
Except my new job was paid in the middle of the month, two weeks in arrears and two weeks in advance. So I was due to be paid on 15th, nearly three weeks money.
But we got paid a day early in my job. I panicked, thinking I was about to lose three a bit weeks' money because it had been paid to me in the wrong UC 'assessment period'. Luckily, the HR person checked and it was reported to HMRC on 15th, which was the day it was due. If they had reported the payment on 14th, their paying me early would have cost me a few hundred pounds in lost benefits. The HR person was horrified when I brought it to her attention and asked her to double-check.
Being paid a day either side of an assessment period can make the difference between someone being paid or not being paid. And when you're on a very low income and/or when you've just started a job and have additional expenses, like cost of commute, lunches, etc, then you can end up losing out and getting into debt.
My current employer paid people early in December. We're usually paid on the last working day of the month, but they paid us just before Christmas, which might've messed up the finances of some of my colleagues for anyone in receipt of UC if they were paid in the wrong assessment period and as a result their income was too high to receive benefits.
It's a terrible system for people who's work payday is out of sync with their UC assessment period and also people with fluctuating incomes. It's good in that you don't have to sign off and on and off and on again if you get temporary work, but it can mean that it sometimes looks like you've been paid twice in some months and so you don't get any top up benefits.
iirc, there was a court case about this anomaly, about how it disproportionately affects single mothers, because single mothers are more likely to be having benefits topping up their income while working.