T304 is the most extensively used of the 300 grade stainless steels. It has excellent corrosion resistence and can be machined and welded making it suitable for use in a whole range of applications. T304 is relatively stable at high temperatures. T304 is not recommended for use in saltwater environments.
It can be bright polished to a high standard of finish and hence is used in structural applications and domestic appliance. It's stability also means it will not change the nature/content of any material it comes into contact with.

The standard finish is fine in situations where the tube is not being used for decorative purposes.  Standard finish material can be bright polished to a mirror like finish but generally it will come descaled and looks acceptable in most circumstances where functionality is more important than looks.

Thin wall material is suitable when weight is premium, or heat conduction is important.  Thin walled tube will still be strong but not as strong as regular walled tube.  It is ideal for lightweight applications and small diameter tube.