To describe the importance of the Great Valley and imply a reference to the events of the first movie the song was quite suitable. The importance of the Great Valley had to be underlined at the beginning of the movie to lay out the stage. It could have been done by a dialogue, but I don't think a dialogue would have gotten the message across as well as a song:
Littlefoot: "This Valley is awesome!"
Ducky: "Yep, yep, yep, it is!"
Petrie: "Me no have worries about being hungry anymore!"
Ducky: "Nope, nope, nope, neither have I!"
Cera: "It was so hard to get here! Now we're safe! Safe at last!"
Ducky: "Yep, yep, yep, we are!"
I admit that one could probably write a more fancy dialogue, but I think that a song was probably more handy to get the message across and to set the stage the message had to be at the very beginning of the movie.