[klaasvakie]

With guitar you are much more likely to have to re-tune during a performance. I can think of a couple of reasons.

Steel core strings (on guitar) have a much higher coefficient of thermal expansion than gut-core strings (on orchestra instruments). If you start a concert at dusk your guitar will go quite a bit sharp if you have a cool breeze blowing across the stage after the sun sets.

Some of the playing styles require large bends or use of a tremelo which is much more likely to throw some of your strings out during a performance. Having a pedal means you can quickly re-tune between songs without having to blast it over the PA.

Also at an orchestra it is expected that the crowd will be relatively quiet so that the musicians can hear their instruments while they are tuning. At a rock concert you will have a tough time hearing your instrument while tuning, and you don't necessarily want to subject the crowd to your tuning noise over the PA.

EDIT: Oh yeah, and I should add - most other string instruments aren't fretted so the musician has the opportunity to correctly intonate a slightly off-pitch string. The frets on guitar means the strings have to be perfectly tuned unless you can bend them in (which isn't really a standard skill)

[analog31]

Actually gut strings are horribly unstable, but are virtually obsolete, having been supplanted by steel and synthetic cores. But orchestral musicians have learned to discreetly tune when they notice that their instrument has drifted out of tune. But like you say, being able to hear yourself helps. The same instruments in a quieter setting are often tuned by ear.