Use Of VPN
VPN abbreviates to Virtual private network.
About the question of legality. Yes, it is legal to use a VPN. But using VPN for some wrong reasons like torrenting will be illegal. Unless you do anything wrong, It is safe.
Even google play store allows such apps.
Now coming to the uses of it. It is used to make your network private by hiding your IP address or any other kind of sensitive information which is used to know your location. If you are not sure that the site you are visiting is safe or not, then you can make use of this app.