Significance of Tanahashi?

My second NJPW match (followed by the one it went after) was Tanahashi’s retirement match. Can someone explain to a non-NJPW fan why this match is so highly thought of, and explain why the ace was so important. Thanks

P.s I thought the match was great

(edit) thanks for the crazy detailed response guys, been very interesting reading through them all, didnt expect this level of detail!

by Jackmanlee