To stop the UK becoming a tax haven when we leave the EU. Where the less well off bear a greater share of the tax burden than the very rich and corporations.
You may be interested in these active petitions
One fear is that Brexit is simply so the very rich and corporations can avoid the EU's Anti Tax Avoidance Directive. They would be able to store their money in the UK and aggressively avoid paying tax. If the UK adopted the EU's Anti Tax Avoidance Directive it would ensure everyone paid tax fairly and give one less reason to object to Brexit