It depends a lot on what is being sold as "topsoil". Topsoil is usually defined as the surface layer of soil - 4-10 or 12 inches - that is present before any clearing, grading or construction happens (most topsoil is gathered from new construction developments). So it could be just about anything - very clay-ey, very sandy, filled with rocks, roots and even various debris! Nothing magical about the stuff :-))

OTOH, it could be pretty decent loamy stuff but is almost always lacking in organic matter - the materials (decomposing plant and animal matter) that provide soil its drainage properties and fertility.

Before I bought "topsoil", I would want to see it to determine if it was the sort of soil I wanted to add to my garden. In my area, it not something I would waste money on. Rather, I would go with a bagged planting soil or a bulk garden soil blend. While these often have topsoil as a major component, it is screened and with some sort of organic matter - usually compost - added. And sometimes with a drainage enhancer like sand as well.

And while many plants will do fine in unamended native soils, many plants would prefer to have a better quality to soil to start. So there is not really a requirement to use just native soils whenever planting but to do whatever amending or adding that might be necessary over the largest possible area rather than individual planting holes.

If there is no place in your garden that you can transport any extra soil from, then by all means go ahead and add some good quality soil. But try to mix it in over the largest area you can and incorporate well into the existing soil also.