A buddy of mine, who served in the Army for more than 15 years, first as an Army Ranger then as a Green Beret, put it this way when I posed the same question.
I'll paraphrase
Infantry men love to have the capacity for full auto fire, even if we rarely if ever use it. It's kinda like carrying a condom in your rucksack, even if you know you won't use it, it's nice to know it's there..... And nothing says you mean business like a quick
'rrrrippp'