Edit: Clearly I’m typing too slowly 
When you have solar panels, your goal is to use them as much as possible. The more power you get from them, the faster you recover the capital expenditure. The best way to do this is to always run them at full tilt, and the simplest way to do that is to push power back into the grid, while getting paid per kwh.
If you can’t do that due to policy issues, import issues, or whatever, your 2nd-best option is to keep your import at zero, i.e. you try to never draw power from the grid, and never push back. You self-consume the power you produce.
So ESS (depending on mode) typically uses power from the solar panels to keep your import and export at zero: first powering the loads, then sending excess power to the batteries, then scaling down solar production. Again depending on the mode, it can use power from the batteries to power the loads, making sure import is at zero above all else (excepting the exceptions, such min SOC, etc.).
To be able to do that, you need a clear picture of what is happening at the grid connection point. You need to accurately see what is happening at that point that you’re trying to keep zero.
Now the Multiplus II has a meter built in, but it has two problems:
- It is not a particularly precise measuring instrument
- It is physically inside the MP at the AC input (there is an extension cable with a clamp too…)
#2 gives the most issues: If your whole house is connected to the AC output, then the AC input is actually your grid connection point, but this is almost never true. Even if it were, you’re still stuck with the accuracy of #1.
So the ET112 and friends give you a very accurate meter that you can put where you want. And you want to put it riiiiight next to your main breaker – the grid connection point.
Now the MP can include loads that are not connected to the AC output, but between the grid connection and the AC input in its calculations. So it will push back power from solar or the batteries through its AC input to the loads connected there, but not so much that the power flows into the grid.
Your big consumers such as stoves, geysers, aircons, etc. are all connected here in most cases.
So the ET helps you power those with excess solar and/or batteries to improve the self-consumption, instead of scaling down production.
It makes a bigger difference the smaller your inverter is, since then most big loads are on the “wrong” (input) side of the inverter.
Hope that helps.