For a unicellular prokaryote (some bacterium for instance) it's easy to adapt to the changing environment because they only have one cell to take care of. For us eukaryotes it's a bit trickier because we have lots and lots of specialized cells and organs. That's when our senses get involved. We need information what's going on inside of our system and outside of our cells. That is one major key to survival. How vulnerable would you feel if all of your senses were switched off? See? I think you got my point why senses are so important.
We can sense many different types of things:
- Mechanical energy (e.g. touch and pressure)
- Electromagnetic radiation (portion of visible light)
- Temperature (hotness and coldness)
- Chemical substances (e.g. flavours, odours and metabolical products)
It could be useful if we had receptors to detect e.g. radioactive radiation or magnetic fields. Imagine the possibilities!