Why is ‘Bearer’ required before the token in ‘Authorization’ header in a HTTP request?

Why is ‘Bearer’ required before the token in ‘Authorization’ header in a HTTP request?

What exactly is the difference between following two headers:

Authorization : Bearer cn389ncoiwuencr

           vs

Authorization : cn389ncoiwuencr

All the sources which I have gone through, sets the value of 'Authorization' header as 'Bearer' followed by the actual token. However, I have not been able to understand the significance of it. What if I simply put the token in the Authorization header?