Helical antennas allow for radio signals of different polarization to be received. At low frequencies, polarization isn't all that important but it becomes significant as the frequencies increase. Microwave signals are sometimes refered to as horizontally polarized, vertically polarized, or circular polarized.

Here are some links about polarization and helical antennas:

Polarization Requirements for Various Frequencies

Performance Tips - Circular Polarization (ham radio link)

Helical Antennas

Helican Antenna definition