Why are nets or foam used to catch people jumping from tall building?

It has to do with momentum.

Ft=mvf-mvi is the impulse momentum theorem where F is force, t is time, m is mass, and v is velocity. Ft is impulse or the change in momentum, which is mvf-mvi. The change in momentum will always be the same, since your mass won't change and you will start with the same velocity and end at rest. The only variable part of this is force and time, since their product is equal to the change in momentum. This is where materials such as nets, foam, blankets, ect. come into play since they increase the amount of time the force is being applied but the force that is applied is much smaller. For example, if you threw and egg at a wall it would break because the time is small so the force is large. But if you threw it at a blanket that someone held up it shouldn't break because the time is longer so the force is smaller.Same goes for your question just replace eggs with people, blankets with nets and foam, and a wall with the ground.