When we look at the world we see a LOT of things which, on the surface, don't seem necessary. At the end of days, we will realize how EVERYTHING was part of Hashem's MASTER PLAN!! That is the meaning of the pasuk "אז ירננו כל עצי יער" - Then all of the trees of the forest will sing to Hashem. Meaning, all of nature will be testimony to Hashem's greatness.
In the meantime though, this is hidden. THAT is what we mourn when we talk about "Galus Hashechina" and will continue to mourn until the building of the Beis Hamikdash and complete redemption.
[עפ"י רזי לי עמ' ע"ז]
