Well, in the simplest reaction you are making #2xxC-C# bonds..........
#"Diene + olefin "rarr" 6-membered ring"#
And as you know, the number of #C-C# bond formation reactions is small indeed. And thus Diels Alder reactions are VERY BIG DEALS. Modern variants of the Diels-Alder reaction have introduced stereocontrol into the reaction, and have also allowed the introduction of heteroatoms.