Apart from getting products on to the shop shelves, keeping a lid on inflation, companies not going bust due to a staffing crisis, not being ripped off with mobile charges abroad, wasting billions upon billions of taxpayer money, visa free travel in the EU, not having to pay extra charges on goods sourced from the EU, generally not stunting our economy and as a knock on effect not breaking up the UK and potentially a civil war in NI. Plus loads of other things I missed...but apart from that. Nothing.
I will say least you are honest with your reasons being solely around immigration and not hiding under the pretence that it was to make the country stronger.