Open terrariums are better suited to plants that prefer less humidity and soil moisture, such as temperate plants. Not all plants require or are suited to the moist environment of closed terrarium. For plants adapted to dry climates, open, unsealed terrariums are used to keep the air in the terrarium free from excess moisture.
Open terrariums also work well for plants that require more direct sunlight, as closed terrarium can trap too much heat potentially killing any plants inside.
Note that succulents, despite being a popular option, are a poor choice for terrarium - open or closed. The intrinsic lack of drainage in a terrarium will inflict or require underwatering.