Sodium and potassium nitrates are commonly used as fertilizers. As they are also strong oxidizers, they enter pyrotechnic compositions and the manufacturing of explosives. 
Eutectic mixtures of alkali metal nitrates are used as molten salts. For example, a 40:7:53 mixture of NaNO: NaNO:KNO melts at 142 °C and is stable to about 600 °C.
A minor use is for coloring the light emitted by fireworks: 
* lithium nitrate produces a red color, 
* sodium nitrate produces a yellow/orange color, 
* potassium nitrate and rubidium nitrate produce violet colors,
* caesium nitrate produces an indigo color.
In a general way, the emitted color progressively turns from the red to the violet in the visible spectrum of light when going down in the column of the alkaline metals in the periodic table of Mendeleev. It corresponds to a decrease of the wavelength of the light emitted during the electrons de-excitation step in the atoms brought at high temperature. The photons emitted by caesium are more energetic than these of lithium.