Sometimes the melody genuinely isn't clear for chords. For example, "Ten penny bit" can be accompanied with Am and G throughout, but Am, G and D fits it better, and changing some G chords to Em makes for a more interesting backing.
Or the tune might be modal, in which case different chords might apply. It might not be obvious whether it's modal or not, in which case you can choose which way to go - "Ten penny bit" again can be done with A (major) instead of Am, and you've got yourself a completely different feel to the tune.
And if you're trying to reproduce what someone's doing off a recording then it might be hard to pick up exactly what they're doing, especially if they're using hammerons, chords outside your basic triad (eg. sus2, sus4, m7), altered tunings, etc. Your typical beginner won't be able to figure out most of this, and some of it might be out of reach of your intermediate player too.
Sure, your experienced player will be able to work it out. But your experienced fiddle/box player won't need the dots - they'll be able to pick up tunes by ear from a recording or playing along in a session. So should we wonder about "tune req" posts, or the existence of sheet music for fiddle tunes...? ;-)