And you find out if the software does what it claims to do. You can usually find out if the software can do what you need it to do. After reading reviews, you can decide which software to demo, saving you time and effort. Reviews point you to where you need to look. The decision about what software to buy is a hard one, especially when the software is expensive.
Some software can cost as much as a used car.
When you go to buy a used car, you check out all the possibilities. You don't just buy the first or even the second car you see. The same principle would apply to buying software. Read what the manufacturer has to say, read and hear what other people have to say, and most important of all, take it for a test drive. Reviews are an important part of learning about particular applications, and a good indicator of whether it can live up to it's claims.