When a polyester fabric has a suede finish, that means that while the fabric is still polyester, the fabric looks like it could be suede. This provides the fabric with a number of positive properties that suede lacks, but the fabric still maintains the look and even the feel of suede.
Ease of Cleaning
Since suede finished polyester fabric is in fact polyester and not leather, it's much easier to clean up and wash. The fabric can often be thrown into the washing machine, though it may require some special settings to be properly cleaned. Regular leather can't be cleaned except by professionals or with particular leather cleaning materials. As such, this property is a major advantage to using polyester material with a suede finish over actual suede material.
Light Weight
Polyester fabric is much lighter weight than suede fabric, whether it's being worn or it's being used to cover furniture. Real suede is made from relatively thick skin, and that skin is heavy no matter what you do to it. Polyester fabric can be altered in weight, and it can be made lighter so that clothing can still have the look of suede--which means it can be worn in the spring or even summertime, while the weather is much hotter than would be comfortable for genuine suede.
The Look for Less
Perhaps the main reason that people will buy suede finished polyester fabric is its look. Suede is an expensive fabric--in fact, leather is some of the most expensive fabric you can buy--and polyester fabric is significantly cheaper. So if you can get the same look and feel of expensive suede while paying less for it--and it's lighter weight and you can wash it easily--that's one heck of a combination. Many people only want the look of suede, and they're willing to compromise on having the genuine article.