EU provides trade, geopolitical weight, smooth proceeded across borders (no passport or customs checks from Poland to Portugal..), reduces friction and competition between the neighbours (first big policy ~60 years ago was the Common Agricultural Policy CAP, which provides common subsidy levels and avoids conflicting subsidies/races to the bottom), enables standards, builds trust in institutions across borders (from passports to taxes to the legal system), provides great trade agreement terms with third countries and visa-free travel to 50+ countries, and most of all it always was and remains a peace project.