A supporting sentence 1. Develops, supports, or explains the main idea of a paragraph
2. transitions ideas from one paragraph to the next
3. introduces a new main idea into paragraph
4. Concludes a paragraph by restating the main idea

They develop the main idea into a paragraph.

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.

It transitions ideas from one paragraph to the next.

Explanation: Because, if you were to support a paragraph to have to use transition words, if you were gonna compare and contrast normal snake and cobras you would say "Normal snakes have no sticked out body from it's head, in contrast, Cobras have a round circle on their heads." Get what I mean?