From what I understand, transcribing songs means writing music down from start to end. Transcribing solos means writing down the solos (of course, captain obvious.) Normally, when you transcribe songs, you should also be able to play through them, with the actual tempo.
Learning solos note per note, across an entire song, provides an idea on how the great artists think during solo - when they breath and when they speak a lot, when they say many and when they say little. The more you transcribe, the more ideas you have - not just licks but entire song ideas - substitutions, lines, rests, etc. Eventually, you can reapply song ideas which may lead to great arrangements, great solos, etc.