From the FAQ:
Since PowerStore was built with eCart, I think it will always help to have it. Powerstore 3 creates all of the tables for you during installation. But that doesn't mean that you can't add columns to the tables.
Think of PowerStore as saving months of work building a fully operational store, that can then be tailored to your needs. Some of the stores that people have built, have no resemblance to the out-of-the-box store. It's what's under the hood that makes the difference.