I obviously have a personal interest in the Brexit 'Remain' campaign as I would like to continue living in the EU! My children will grow up mostly Belgian but a bit British and some German influence, and so for them the reality of a fluid Europe where there are no borders and where people mix will be very real.
So I would be sad for the UK to miss out on that. Many people have seized the opportunity to be part of the EU, and have been on ERASMUS programmes, lived abroad, learned languages etc.
"I honestly cannot see how the UK would benefit from 'Leave'"
But many Brits still live with an 'island mentality', speak only English, see 'The Continent' as something far off and homogeneous. I fear that leaving the EU would only widen that gap and make the UK even more insular.
From a 'material gain' point of view, I work in customs and I honestly cannot see how the UK would benefit from leaving the world's largest single market.
Many politicians cite Norway and Switzerland as examples without understanding that they do not have the same rights as EU countries within the common market, and they have far less attraction for international companies as a European base than an EU country does, for obvious trade reasons.
"The good outweighs the bad"
I understand why people in the UK see the EU negatively as the UK media have used it since a long time now as a scapegoat. Unfortunately many of the positive things have not been very loudly communicated.
That a lot of money is spent is certainly true and that some reorganisation and rethinking is needed is also true, but you certainly can't do that from outside. I think there is also a lot of complacency in the UK that we do not need the EU to keep peace. We have forgotten all that came before and the reason why the EU was created in the first place.
To summarise, I do not think the EU institutions are perfect by any means, but I do think that the good outweighs the bad, and that UK needs the EU probably far more than the EU needs the UK.