A lot of companies use hot air balloons as a means of advertising or promoting their product. This is is a great form of advertising as when a person sees a hot air balloon from the ground, they always tend to look up as it is something you would not normally see everyday. If when the person looks up and sees either an advert on the side of the envelope or the actual envelope in the shape of the product, then they are likely to remember the product as it is being displayed so large in front of them.

Here for example is a financial times shaped balloon envelope:

Balloons that have envelopes like this are referred to as 'Special Shape' balloons. On the picture below you will clearly see how an actual balloon company advertise their flights by having their name incorporated into the envelope:

Banner on Envelop Advertisement