The solar panels generate what is called DC or direct current, however the mains supply you have in your home is AC or alternating current. The purpose of the inverter is to use electronics to convert the DC into AC.

The AC produced however, must be at the same frequency and also in sync with that of the main supplies.

It should be noted that at night when the solar cells are not generating electricity the inverter will still require some power. This power usage will lower the overall efficiency of the installation.