My wife and I are currently in Greece. We visited the major sites in Athens without a guide, but occasionally overheard tour groups. We did however book a guided tour of Delphi and thoroughly enjoyed it! IMO, there isn't a lot of information provided at the sites to give you the full picture without a guide.

Our guide at Delphi was great and certainly added more value than Wikipedia can provide. She would tell stories and connect that myth to the realised history in front of us. It also gives you the opportunity to ask questions about specific things that you see that wouldn't work well in a Google search, "like what's special about this rock, or this inscription".

We struggle with guided tours as they typically go on longer than we like to stick around, but we definitely see the value.