Some sort of reflector behind a spiral antenna
In general, ground planes on antennas are used to effectively double the length of the antenna. This is done by the properties of what we call &quot;Image Theory&quot; or &quot;Method of Images&quot; and can turn a quarter wavelength monopol into a half wavelength dipole.

However, in the case of endfire helical antennas, such as the one you are referring to, the exact geometry of the ground plane can have a significant effect on the gain (to the tune of ~4dB) as documented in this paper.