These are IR sensors called breakbeam sensors. They act as an invisible, electronic tripwire. I could use these to make a light path for instance. I could connect these with long strips of LEDs and have them line a small walkway so that when someone begins to walk on it it would light up.
This sensor can detect several gases, however it can’t tell which one is present. This would be useful in a chemistry setting. I could make a sensor that someone could keep around so that when they are working in the lab they would be able to be notified if a toxic gas became present.
This is a soil moisture sensor. I could use this to make something similar to a project I saw at a hackathon. Someone made moisture sensors and used a program to convert the signals into different audio clips allowing the plants to “communicate” with you.