When someone is going through fabric shops with the goal of finding fabric for a specific project, they need to know how thick or thin they want the material to be. The one who is purchasing fabric to be used to sew clothing together has to know how warm and how durable they want that clothing to be. If a person is unsure what they should be buying for the sewing project in front of them, they may be able to get help from the person who is running the fabric shop that they are shopping at. Such a person might be able to tell them what they recommend for the type of project that they are taking on.
The one going into a fabric shop is going to see a variety of material in front of them and they might not know what they should purchase or how much of each type of fabric they are going to need. The one who is working on a project that does not tell them how much material to buy might talk to those who are working at a fabric shop and see if they can help them figure out how many yards of each material they are going to need.
If a person is just getting started buying fabric and working with it, they should be willing to talk with the person who is running the shop that they are using and see what advice such a person has for them. Many of those who are working at fabric shops know a lot about sewing and about the types of materials that are needed for different types of projects. The person who is willing to get advice from someone who is running a fabric shop will end up satisfied with their purchases.