Hi,
That's a filter used to prevent unwanted signals from reaching the digital part of the circuit. Some frequencies could align with the switching frequency of the switching filter, which would mean the switching filter would not work correctly on those signals.
The filter is used to limit the bandwidth so that never happens. That way any signals that reach the switching filter are within the right range that it can work with.
Switching filters assume a certain degree of signal smoothness at the input.