This isn't completely true. They are more accurately considered to be letters of intent. Both sides benefit. There's no downsides.
The manufacturer appears to have a popular product in demand and on back order - this creates market and undermines competition. It also helps protect "list price" because a false impression of demand is created.
The airline appears o be positive and expanding. No cash has changed hands and no manufacturing started. It can be hype where all but competition benefits. In fact another plus for those involved is it puts pressure on competition.
Things aren't always as they seem ......
Indeed - VS's order for A380s for example