Not every job is glamorous and well recognized. After all, we don't often think to compliment the person collecting trash on our street, yet if he mysteriously disappeared we would quickly notice.
Maybe your job is thankless; you're invisible to those around you. Perhaps you could only give half your effort and no one would be the wiser. Why not then? Should you give 100% if no one will see a difference?
But the amount of effort you give has more to do with you than how it affects others. It speaks to your character, your integrity. Whether you are leading thousands of employees, or stocking the farthest shelf in the supermarket, take pride in what you do. Strive for excellence in the little things, even if you are the only one to notice.