For mutual support and ease of doing business in specific areas of interest.
Border control? We've already got it for non-EU migrants, and for EU migrants through negotiated opt-outs. In terms of workers, the UK can't discriminate against EU workers on nationality grounds, but employers can still set requirements re skills, English proficiency, etc.
Legislation? EU law applies to EU-wide issues, and UK law to non-EU issues. The UK supports most EU law and negotiates opt-outs and similar mechanisms for other parts. Trade is, of course a massive component in this, but EU law has also helped to protect things like workers' rights and environmental standards.
Lawyers - In For Britain did a dstraightforward summary of misconceptions:
http://lawyers-inforbritain.uk/b-m-a/summary-of-misconceptions/
There's a massive scare-story industry in the UK that regularly trots out headlines of the 'EU trying to steal our borders/jam/whatever!' The reality... the EU almost certainly isn't.