Do horses NEED horsehsoes? if so, why?

I've owned horses for 3 years now, and my parents are totaly clueless about horses, so they trust my trainers word on EVERYTHING. we recently sold our old horse and the lady who bought him took him barefoot because he has navicular, he seems much happier now and is never lame. my trainer grumbles about how a horse HAS to have shoes and buddy was going to go lame any day now. I think this is just because her husband is a farrier and the money from that helps run the farm. We bought a horse who was happily barfoot and my trainer went right ahead and put shoes on him, my parents didn't care, they said she nows best. he has no problems and healthy hooves, when i asked her why he had to have shoes on she said "because he does, horses need shoes, end of story, they just do." I'm confused and no one will lisen to me, he seems more uncomfortable with shoes on and i dont get why her "has" to have them. do horses really need them? if so, why?