If you land upon spikes from any height you will die instantly. Because of this they can be used a more reliable form of a trap compared to lava. Spikes will disappear if they're placed in water, so ensure that there is no water in the vicinty, and make sure you don't flood the area where they are placed, otherwise the spikes will be destroyed. The same applies for lava.
Steel spikes are harmless while walking around them, but not falling on them.