All the reasons for having vinyl tops on cars are valid, and it goes way back to the horse and carriage days.
The reason you have bubbles are many fold, bu the basic reason is that moisture has gotten in between the vinyl and the underlayment, destroying the glue bond. When that happens, oxidation of the underlayment and the backing on the vinyl as well will cause the bubbling effect. If your vinyl looks taught to the underlayment but you can discern a bubble under the underlayment, water leaks have caused rusting of the roof panel itself.
The concept of vinyl or leather coverings, known origionally as a landau top, was indicitive of economic oppulance by the owner of said vehicle, and putting vinyl tops on common vehicles, gave ego boosts to owners, deluding themselves that they were driving a higher end vehicle than actually possessed. The fact is that Vinyl roofs often do enhance the eye appeal of lots of vehicles. Its also a good way to cover up a butchered top chop, or roof repairs.
Having done automotive upholstery off and on over the past 21 yrs., I have had the unfortunate circumstance to run into a myriad of reasons for vinyl roofs becoming unglued.