there are benefits such as child tax credits which are not tapered. You either get the full benefit if earning £99,999 a year, or nothing at £100,000. This benefit if you have children can be worth tens of thousands of pounds, thus resulting in a net loss. If you look at income data after tax this causes a weird drop in take home pay in the roughly 100-130k range, where people just salary sacrifice all their extra pay to make sure they are under 100k. This is not productive to the economy.