Reaper King Scarecrows don't just scare away the birds, they also scare away humans
They serve to give your side of the board tempo by allowing your creatures to attack (removing defenders), giving you more time to set up the board (removing attackers/expensive creatures), and allow for combat tricks (saving your creatures after damage is on the stack
But as the metagame tilted toward graveyard hateparticularly following Aetherdrift 's Ketramose, the New Dawnthose cards became increasingly fragile
In fact, you might even be casting it to protect your stuff from your own land destruction or board wipes
Even then she's pretty good
You cast a colorless spell (hopefully an Eldrazi), you deal a damage, you make a Spawn token, you deal another damage