Bandpass filter BPSK
The filter that you're referring to is called a preselection filter. Its purpose is to filter out everything but the desired signal of interest before mixing to baseband. Unwanted components could include other signals that are nearby in frequency, or just noise that lies outside the desired signal's bandwidth. 

Preselection can serve multiple purposes:


It is common to have a low-noise analog amplifier in your receiver chain to reduce the receiver's overall noise figure. Bandlimiting the input of the amplifier using a preselector can result in improved amplifier performance in some cases, especially those where there are strong interfering signals in nearby bands. 
In cases where bandpass sampling is used (i.e. the desired signal is sampled directly at its nonzero intermediate frequency), a bandpass filter will typically be required in order to remove other signal or noise components that would otherwise alias into the sampled signal bandwidth. 


Whether the filter is optional or not would be application-dependent.