As for the latter, the wish is to have an open-air, convertible-like driving experience coupled with weather protection. A major downside is the creation of a greenhouse effect, whereby the passenger compartment becomes overheated in sunny conditions. Use of air conditioning can counteract that, but at the price of reduced fuel economy.
Back in the mid-1950s when air conditioning was a rare, expensive accessory, Ford Motor Company launched models featuring a transparent roof over the front seat. That roof was a thick acrylic glass, as this source mentions. These panels were given dark blue-green tints in an effort to block sunshine effects.
Nearly 23,000 1954 model year Fords and Mercurys thus equipped were sold, and the feature was continued for the next two model years amid declining sales.