The need for network security is quite obvious, (no offense to the asker), but, it is simply thus: There are criminal activities in every field, computers being no exception. PEOPLE like to store private information on computers. If a criminal was able to slip onto your network, they WOULD be able to access any unguarded computer, and retrieve information off of it once they have access. MAKE sure you keep AT LEAST ONE password on every computer you own, MULTIPLE different ones if it allows it. 

The need for network security is quite obvious, (no offense to the asker), but, it is simply thus: There are criminal activities in every field, computers being no exception. People like to store private information on computers. If a criminal was able to slip onto your network, they would be able to access any unguarded computer, and retrieve information off of it once they have access. Make sure you keep AT LEAST ONE password on every computer you own, multiple different ones if it allows it.