Research shows that encouraging students with external rewards can help a student accomplish a short term, concrete goal such as getting an "A", but usually increases fear of failure and decreases intrinsic motivation which interferes with long-term learning success. Students ultimately must have the interest and drive to learn on their own and helping them realize and find ways to apply their true interests is the first step. The exception is in developing automaticity with foundational material like learning to read, spell, or master math facts. These are challenging and essential skills for young children and sometimes small rewards are essential to keep them on the right path.
You can read more about the importance of developing intrinsic motivation in these Articles on Grit, Growth Mindset, and Motivation.