Yes, the PLL is essential in order to connected the DC/AC inverter to the grid.

The PLL is almost universally used for synchronising an inverter with the grid. There is research, however,  on alternative ways of synchronisation without the use of a PLL. The motivation for this is that the PLL does not work well on very weak systems, or when there is a disturbance in the grid. 

You don't need PLL if you are using virtual flux based control such as Direct Torque Control (DTC) or Direct Power Control (DPC). ABB has made DTC based grid side inverters since end of 90's when first ACS611 frequency converter was placed on the market.

PLL is a preferred technique while synchronizing with grid, line commutated inverter is also a challenging and interesting one..... If our generating power is very higher than connected load or  grid means PLL is not necessary.....

per today`s research, near "Grid Following" inverters, "Grid Froming" inverters are being tested where the "virtual inertia" will be determined by the "smart" inverters.

PLL is not the only control technique compared to;

1) Droop-Based Grid-Forming Control Methods

2) Power Synchronization Loop

3) Voltage Controlled Inverter

4) Virtual Synchronous Machine

5) Virtual Oscillator Control

6) Direct Power Control