If you are applying polyurethane for the first time the task can be difficult. Polyurethane must be applied in long, even strokes. You must also keep a wet edge.
If you touch an area with your brush that is dry or drying, you’ll pull the finish, which could leave bumps when it finally dries. These bumps will need to be sanded. You’ll need to lightly sand cured layer to apply subsequent coats. This, once again, entails additional time and work.