Two reasons, one covered and one not.
1. Its easier to remove the tile and then put it in smaller tank to hatch the eggs.
2. You can replace that tile right away so you don't change the clownfishes environment. The goal here is to make the clownfish feel like nothing happened. If you have to remove the whole rock it laid eggs on you might make it feel like its environment is changing which in turn will make it stop sawning.
David
BTW, I love the clowns in your avatar!