In a grid attached, net metering, no battery system, microinverters on the roof can then basically feed directly into the breaker panel and you have to do minimal rewiring of the house to get it up and running. This is why they exist, for quick retrofit of a house without having to install an inverter inside the house. They also shutdown with loss of grid, so meet the rapid shutdown rules without needing extra controls buttons, etc. Another aspect is that the panels system can be any number of panels, no balancing required, just add panels with microinverters.
The partial shading benefit and redundancy if a panel breaks do exist, but I don't think that's the primary reason.