Yeah it would be great if nobody agreed to commit any violence ever.
But until that point, it's nice to have some soldiers on our side who can liberate your country if the other side does decide to fight, or special forces to rescue you if you get kidnapped in a failed state by some people who decide to fight. And let's hope they're intelligent, disciplined, responsible and care about your life.
If you had been a young man during WW2 would you have fought beyond the UK's borders to liberate Europe from Germany?