It does help them. I use a 50% shadecloth using PVC pipe hoops and EMT posts to suspend the shade cloth up high over the plants. I usually do it once we're hitting the upper 90s, but haven't put it up yet. I need to do that. Be sure to leave a little air space between the top of the plants and the bottom of the shade cloth---about a foot of air space works for me. If the plants are too close to the cloth and there is poor air movement, the plants can be roasted, toasted and cooked. Some years I shade plants and some year I don't. Usually it just depends on how pitiful they either do or don't look once the heat sets in.

In my garden, though, if I have to choose, I'll shade the pepper plants and let the tomato plants deal with the sun on their own. With peppers, it is so easy for the fruit to scorch in the sun's direct rays that shading them is usually a bigger priority for me than shading the tomato plants. However, if a plant has lost a lot of foliage due to disease, that can leave the tomato fruit vulnerable to sun scald as well, so shading them would be important if that were the case.

Just remember that, depending on the density of whatever textile you use, you can be cutting the sunlight enough to slow down the fruit development. I usually try to work around that by having the shade cloth directly over the tops of the plants, with the sides of the plants still getting quite a lot of sun. It is the best of both worlds. Other than shade cloth, back before I even had shadecloth or floating row cover, I used old bedsheets or even old sheer curtains (they may be sheer but they block some of the light).

All kinds of animals and birds like shadecloth and may decide to hang out on it and around it, or even in the shade beneath it. Mine has served as cat hammocks, snake sofas and bird perches.