Answer:
They develop the main idea into a paragraph.
Step-by-step explanation:
The supporting sentence of a paragraph essentially summarizes what you are going to say in the paragraph into one sentence. For example:
If you wrote a paragrph about sloths, a supporting paragraph for how they survived may be, "Sloths have survived through evolution because their lack of bathing is unnapealing to predators." The rest of the paragraph would elaborate on this statement.