Frequency and period are reciprocals of each other. CalcTool is able to handle reciprocal units within a single menu, so this calc may be used for frequency-to-frequency, period-to-period, or interconversion between the two.
Frequency and period are used to measure properties of waves (e.g. sound, light, ocean waves) as well as other predictable events. Events that happen aperiodically but with a predictable average rate (such as radioactive decay) are often measured in Bequerels (Bq).