A water wiggler might be that toy or it may not. Not sure what you are looking at. For this use a water wiggler is something the size of an egg filled with gelatin or water and you stick a thermometer in there. That way you can tell what the temperature is in the middle of an egg, not just measure air temperature.
In a forced air incubator it doesn’t matter that much, but in a still or thermal air it can. Hot air rises. You can get a lot of temperature change just by going up or down a bit. For example, you want an incubating temperature pretty close to 99.5 at the center of the egg, but in a still air the recommendation is 101.5 at the top of the eggs. Supposedly that gets you pretty close to 99.5 at the center if you have normal sized eggs and you are incubating them on end, not lying flat. Some people use water wigglers to control what is actually going on at the center of the egg.