While the main Benaki Museum of Greek Culture displays the Classical and traditional, this annexe focuses on modern and inventive. It hosts only rotating temporary exhibits, so whether you go depends on whether you like what's on at the moment. Also check the schedule of musical performances in its courtyard. It has a pleasant cafe and an excellent gift shop.
If you're planning to visit more than two of the Benaki museums, consider the €25 Benaki Museum Experience, a pass that is valid for one visit to each, over three months.