Just about everyone tunes their guitar with a tuner these days, it's true. The only person I've ever met who NEVER uses a tuner for guitar is my cousin. His ear is better than any tuner I've ever seen, but for me ............. Tuner.
A lot of the reason for that is that you're rarely in an environment that's QUIET enough to tune well by ear. That's jams, gigs, rehearsals, etc. Mostly too noisy to get a good tuning of an acoustic or electric guitar by ear. That's also why I like a tuner that picks up your guitar's vibrations instead of through a mic.
For TRUMPET, I like the (KORG) tuner pretty much only for figuring out quickly what key we're in when playing along with CDs, the radio, and movie sountracks.