Insurance.

You want your LiFePO4 to absorb the day to day brownouts/outages as well as sunlight variation and charge/discharge, and you want your lead acid there for when you get taken off the grid for 3 days and maybe have your solar ripped off your roof or partially degraded, once every ten years.

You also can afford to capture more energy faster and charge your lead acid slower - vs just dissipating the extra the lead acid can't use as heat.