word usage - "Many a year" vs. "many years"

so soon they were all seated at Beorn's table, and half had not seen such a gathering for many a year.

Why is many a year used? Should not many years be used?
What is the difference between many a year and many years?

Answer

"Many years" has always been the preferred and more grammatical usage:

"Many a year" as barbara points out is almost exclusively used as a poetic or stylistic variant, but in normal English you should use "many years" instead.