The suction cup is used to remove large sections of glass.

Thank you Frank. But she already had a separate glass cutter (a long item with a blade). Wouldn't that cut out large sections of glass -- as large as a whole window pane? Why would she have to cut glass in circular shapes?

My question is, if she was having an affair with Harold, wouldn't she have access, or know how to get into Harold's home? Why would she even need the glass cutter?