Off Grid Inverter If you are using solar charged batteries for your power you need it.
Grid Tie or On Grid Inverter If you use the power grid (electric company) to buy or sell power you need it.
Why do both? The solar electricity you generate is in the day light hours. Youi can sell your excess to the local power company during that period. Instead of buying back from the grid at night or cloudy days you can use batteries. The batteries are also available as emergancy backup if the grid goes down. What do you do if the batteries run down when the sun is not available? As a fail safe on batteries running down many people buy a propane based generator to recharge the batteries when they drop below a certain percentage. The generator runs for small periods of time just to recharge the batteries. So as you can see, an Inverter that does both on and off grid gives you a full slate of options for energy management and independennce.