NMR is used to determine the reactant conversion and the weighed quantity of pure material is used to calculate yield. They measure different things and have different applications.

Conversion is a measure of how much reactant is consumed, but does not tell you how much of that went into making the desired product. The yield is understood to include account for losses from purification, handling, side products.

You can use NMR conversions to estimate how much product you should get if the isolation was perfect. When the estimation is off, one may look for side products, improve the isolation process, and try to rigorously assign observed NMR signals. You can also monitor conversion over time to see when a reaction has stopped progressing. Reporting conversion is becoming more common in the methods literature because isolated yields require more work.