You probably chose the “do not claim the tax free threshold” option on the intitial paperwork. I did that when I first started working - I didn’t know what it was, so figured I shouldn’t claim it. It did lead to some lovely tax returns until I told payroll to change it, but these days with high interest in bank accounts available it is not such a good strategy.

I use automatic payments to divide my pay up into a bunch of different accounts, so the money that I have available to spend is separate from the money put aside for bills or savings goals. Having everything divided up into smaller amounts earmarked for specific spending makes me much less likely to spend it on something less important than having it all in one place.