The UK provides a disproportionate share of the EU budget. It is akin to the US's 25% of the UN budget.
For the UK, it is a chance to break free of crushing regulations and the danger of uncontrolled Muslim immigration because Merkel felt guilty for WW2.
For the EU, it risks losing one of the biggest sources of revenue, hence the hysteria from the remain camp to prevent that. As well as the politicizing of the death of a "remain" camp MP murdered recently; she wasn't killed for her politics but was killed after intervening in a madman's fight with another. But he was conservative, she's an attractive mother and remain party, so she's already been made a martyr for the "remain" campaign to try to guilt trip people into staying, though the same media covered up the Rotherham rape scandal and other crimes by Muslim migrants because THAT is against their political agenda.