As others said - they could have been talking points for a classified meeting. Or maybe it was classified info that she was trying to use as background to formulate her talking points. Or perhaps it was non-classified material that just happened to be in the classified system.
Question: Does Clinton have the authority to make that determination for herself? Can she just order a subordinate to copy something out of the classified system into her nonsecured E-mail, and to strip the classification headers from it, based solely on her own judgement that the material shouldn’t be classified? Or was she supposed to seek permission or notify someone of what she was doing? I really have no idea.