Well, for example, if you had a 1kHz high pass filter or something with a slow rolloff (actually, this makes more sense as a smaller value capacitor in parallel with a higher value resistor inline with the signal input), then frequencies below it would be attenuated, then you could take the result and amplify it (or, amplify it and then filter, which would probably be lower noise). I had been thinking preemphasis in the digital communications sense...
A capacitor in series with the signal is a high pass filter on its own, so adding a resistor in parallel to it lets some of the lower frequency and DC content pass. Adjusting the value of each adjusts by how much - lower value capacitors pass less low frequency content, lower value resistors pass more DC current but also allow more of that low frequency through.