When to use solar DC optimisers

When extending strings to different roof orientations or combatting shade, optimisers are useful. In Australia we typically use Tigo optimisers for most inverters, Huawei, now iStore for those inverters, and SolarEdge optimisers for SolarEdge inverters.
Ignoring SolarEdge because their entire system works on a fixed voltage which means their optimisers must be able to lower (buck) or raise (boost) the panel voltage, the other optimisers Tigo/Huawei/iStore work in 'buck only' mode.
They lower the voltage in order to increase the current in keeping with Watts law (P=IV) or Power = Volts x Amps (Current).
With string inverters, there can only be a single flow of current down the wire. Optimisers make sure that the current produced by the others panels on the string is not reduced. They do this by lowering their voltage to match that current. That's why they can be so helpful in multi-roof orientation installations and with handling some shade.