Stretching lengthens your muscles, so for a given contraction, you get more work when your muscles are stretched out. Stretching consistently over time, widens your muscles, so they have more pull when you use them with less chance of injuries. If a muscle is tight, you can only contract it so much. Let us take the bicep muscle. If there is tension in the muscle, and it is tight, by let's say, 20 percent, you cannot use that 20 percent when you contract it to lift something.
However, if there is minimal tension in the muscle, you can use more of the muscle to lift something. Stretching can help you utilize what muscles you have to do more work.
So, stretching makes you stronger.