The nanowire array (NWA) layers with controlled structure profiles fabricated by maskless galvanic wet etching on Si substrates are found to exhibit extremely low specular reflectance (<0.1%) in the wavelengths of 200-850 nm. The significantly suppressed reflection is accompanied with other favorable antireflection (AR) properties, including omnidirectionality and polarization-insensitivity. The NWA layers are also effective in suppressing the undesired diffuse reflection. These excellent AR performances benefit from the rough interfaces between air/NWA layers and NWA layers/substrate and the decreased nanowire densities, providing the gradient of effective refractive indices. The Raman intensities of Si NWAs were enhanced by up to 400 times as compared with the signal of the polished Si, confirming that the NWA layers enhance both insertion and extraction efficiencies of light. This study provides an insight into the interaction between light and nanostructures, and should contribute to the structural optimization of various optoelectronic devices.