"De" is used to connect another preposition (pe here) or an adverb to a noun. The sentence wouldn't be correct if it only had one or the other.
I wrote "The fish ON the table is good" and it was accepted, but the suggested solution is "The fish FROM the table is good." In the statement "Peștele de pe masă este bun", is it clear whether the fish is actually on the table right now, or can it mean both that the fish is on the table and that it has already been lifted from the table?
Interesting, mine was drag the words up (no free-style) and "on" wasn't a choice. I would have thought "on" myself, and was surprised it wasn't there.