Without light we wouldn’t have cameras! 🙂
A flash can allow the photographer to take high quality photos in conditions that normally would make it impossible. Since we cannot always control the light a flash becomes a required accessory for many.
Minolta made many flashes and like their lenses Sony reproduces some of them changing only the name on it. Minolta flashes such as the 3600 and 5600 will work well on modern Sony dSLRs and older models can be used as slaves or even on camera flashes with limited feature sets.
Maximum flash strength is measured in GN or Guide Number. The higher the number, the more powerful the flash. Sony encodes these rating into its product name. i.e. Sony HVL-F58AM is GN 58. Read more about guide number at Wikipedia.