Being in the European Union provides several advantages to both businesses and individuals in the UK.
First of all is the freedom of movement of goods, services and capital which basically means that if a service or product is bought by a UK company from another EU country or vice versa, they will not have to pay import duty (a tax) on these things. This makes it cheaper for UK businesses who buy raw materials from abroad, therefore keeping their costs down. It also makes it easier for businesses to sell to other EU countries (eg. if you are in Belgium and have to choose between a supplier inside the EU and one outside that sell their service for the same price, you would not have to pay import duty for the EU supplier so that would be cheaper). The fact that the other EU countries are our largest export and import partners means that the UK could be hurt by Brexit, whilst making prices of goods in the UK higher for consumers.
Secondly, there is the freedom of movement across the EU. This means that if you live and one EU country and want to live in another, you don't need any permission or documentation to do this. If the economy were to crash in the UK and we were still in the EU, you could easily just go to another EU country and get a job there (the same applies if you want to go to university in another EU country). This also guarantees visa-free travel as well as some healthcare cover when holidaying in the EU.
Thirdly, there are the laws made by the EU. These laws include vast amounts of human rights laws and regulations as well as a lot of consumer protections. If you think that the Tories will just pass these into UK law, then think again. Other laws include regulations which protect the environment and reduce pollution and greenhouse gas emissions. Police forces in different countries can also cooperate in order to make it easier to catch criminals who operate in multiple EU countries.
Some of the many other things that the EU has done include recover dwindling North Sea fish stocks, provide struggling farmers with subsidies to help them and increase scientific advancement by giving subsidies to universities and companies and facilitating cross-border cooperation and help poorer regions within EU countries with extra funding.
Whilst I acknowledge that there are many problems with the EU including high costs, a bureaucratic system as well as a tendency for a large amount of people from poorer eastern European countries to move to richer western European countries, I personally believe the the UK would be better off staying than leaving even if the EU is in desperate need of reform.
I hoped this has helped you understand