I have been searching for this 'good reason' to stay in the EU for years. Still waiting.
But I know why the politicians want us in.
Traditionally Northern Europe was rich and southern Europe was poor.
The poorer southern countries of Europe needed huge investment in their infrastructure to create modern, richer economies that could buy goods from the richer Northern European countries like Germany and the Uk.
The EU was seen as a good way of redistributing money from those richer Northern European countries to these poorer European countries, and that still happens. That's why some EU members are net contributors to the EU and some are net beneficiaries.
So the UK pays in £350million per week (pre rebate) and the EU uses some of that money to support the economies of the EU members that are net recipients of EU money. It also uses UK tax payers money to give to the next set of countries that are waiting to join the EU to bring their instrausture, motorwats etc up to modern standards, before they join the EU.
So it's a form of socialist redistribution of wealth. The only way that can be enforced is to have a set of rules that all member countries have to abide by - like letting the EU make the trade deals, instead of each country making their own to suit their own countries need. That means that the European countries that we used to compete with for trade and markets suddenly became our 'EU partners'.
But to get people to agree to this you have to have some 'rewards'. For the poor countries it's easy - they get free money from the rich EU countries and their workforce gets to live and work in any EU country that want to which pays higher wages than their home country. They benefit.
For the rich countries the employers get unlimited amounts of cheap labour from the poor southern countries, which allows employers to keep wages down.
Win, win?
Not quite. Because as poorer EU countries get their improvements and their populations move to the richer countries to get work, the employers benefit, but it's the taxpayers like you and I who must find the cost of the additional public services that are needed to support the very large incraese in population that the migration presents. So the tax payers has to find the money for new schools, hospitals, homes, motorways and all the other infrastucure that's required. That's why rich people like the EU - they gain, while the tax payer picks up the tab.
But to keep you shackled to the failing EU they pretend that if you leave you will be denied access to their 'single market' and your economy will collapse (Osborne's Project Fear). Of course this is not true because 168 other countries in the world that are not in the EU manage to trade quite happily without being in the EU.
So, the EU is the biggest confidence trick ever perpetrated on the Uk and the British people need to stand up and say 'enough'. It's ruining the country we love.