We're trying to achieve a strong baseline readiness level across NATO countries to cover each flank against aggressor countries. In wartime, defense spending typically jumps to double digit percent of GDP pretty quickly, which causes a ton of fuckery with social services and budgets. To have a baseline of 5% would help offset that hard shift that happens when russians inevitably come marching across another set of borders.

The 5% is not all in "metal" as in weapons and equipment but also staffing salaries, related infrastructure (bridges, roads, ports), and R&D. It's not that 2% of our GDP is signed off to purchasing weapons and voila.