Hmmm . . . maybe it'll help if I try to outline some stuff here.
One of the remarkable things about
Arda is that it does not necessarily
need light to exist. Sure, things need the light to grow, but many things slept during the times of darkness that occasionally enveloped the lands.
First
Yavanna planted the seeds of the
plants on
Middle-earth, and then she created the two lamps, Illuin and Ormal. After the light from these two lamps was created, things began to grow. Then
Melkor came and destroyed this first light. This ended the Spring of Arda.
The
Valar then moved to
Valinor where Yavanna sang the two
trees into existence: Telperion and Laurelin. These two trees signified
the blessed light of the heavenly
realm of the Gods, so to speak. They signified the beginning of the count of time, and also the beginning of the Days of the Bliss of Valinor.
The light of the two trees never really reached Middle-earth except through the stars. This is why the
Elves were born in the starlight.
Now, when
Feanor created the Silmarills and locked within them the light of the two trees, he created probably one of the most beautiful creations ever to be created.
For Feanor, being come to his full might, was filled with a new thought, or it may be that some shadow of foreknowledge came to him of the doom that drew near; and he pondered how the light of the Trees, the glory of the Blessed Realm, might be preserved imperishable.
The Light of the two trees was unmatched . . . it was the Days of Bliss. So, within the jewels was locked this light, this treasure.
When Melkor destroyed the two trees he, in a sense, put an end to the Days of Bliss in Valinor, and darkened the most beautiful and hallowed light that ever (or will) existed in Arda. The Silmarills were treasured because they contained that light. They are important, as has been stated earlier, because much of the
History of Arda was woven about their fate. And they came from the two trees, thus making those two trees important in yet another way.
I hope that helped.
And as a side note, the Eldar are actually the Elves that went to Valinor and saw the light of the two trees . . . so there's another connection!