It’s not just earners up to £12K but benefits those who are on low / middle earnings the most as they keep a higher % of their overall earnings i.e. the policy put more cash in the pockets of the majority of people and gave them more spending power for things like houses. In fairness, it was the Lib Dems who pushed it through and the tories stuck with it.
I can tell you from experience that the removal of working tax credits had a far greater (negative) impact that the changes to the tax system for low to middle earners. The policies of the tory governments have unequivocally NOT put more cash into the pockets of the majority of people, and if you believe that they have, you are deluding yourself.