Squarespace is a website builder first and a store second. Its templates are the best-designed in the mainstream builder market, and the editing experience is coherent in a way that competitors struggle to match.
For a small catalogue, a service business selling a few products, or a brand where presentation matters more than merchandising depth, that focus is an advantage.
It is not built for large catalogues, complex shipping logic or heavy third-party integration. Merchants who need those things outgrow it.