The point is that it ends the UK having to accept all laws made by a largely unelected government in Brussels, it ends the requirement that the UK has to accept every immigrant from EU countries without question so we get control of our own borders back, and as the UK is a net contributor to the EU budget, it saves money. Not as much as the Brexit campaign alleged, because a lot comes back as EU grants, but some.
And the clear future plan is to achieve a US of Europe by stealth. That idea fills the average Brit with horror. Best to get out now. It's far more than just an alliance, it's giving up independence. We already have to have "European Union" on our passports and ever since 1992 we are EU citizens as well as citizens of our own country. It doesn't take much imagination to see where this is going. The rest of the EU is not so bothered about national borders as they've shifted around a lot in the past anyway, but ours have almost never changed because of being on a couple of big islands.
The downside is that free trade with the rest of the EU will end, but that's hardly the end of life as we know it - a trade agreement can be negotiated, in fact it largely has been, and it frees the UK to negotiate its own deals with the rest of the world. Former Commonwealth countries like that - the Aussies were the first to jump in "hey, let's get talking". EU countries can't do their own trade deals, Brussels has to agree them for the whole EU. But a benefit of Brexit is that we'll take control of that again. Just like before 1973.
Rather a big point - when she took office, Theresa May set up TWO new Departments, One for Exiting the EU, and also another for International Trade. Of course there is one that covers trade already, but she decided a whole new extra Department was needed for all the extra work.