It was designed to do 4 things:
1. Give the silo something else to do once you've cleared out everything in range. You might want to not reclaim it so you can keep a couple napalms on hand for any armies. So you can at least make siesmic missiles (which have 2x the range of all the others) and get some strategic advantage from the investment.
2. Detect cloaked units. It doesn't do this well (or at all) right now because of how cloak reveal-on-damage is implimented.
3. Counter terraform walls from a long distance away, especially on FFA maps. Terraform was used much more aggressively when it was implimented, to block-in superfusions, mohogeos, antinukes, and entire bases.
4. Offer a cheaper broad-area terraform option with more. Because terraform can do -anything-, it has to be fairly expensive. But since all this does is smooth, we can offer a strong terraform option at a discounted rate.