Flashers for emergency warning lights have been around almost as long as the lights themselves. The obvious ones are halogen flashers and strobe packs (which contain transformers and flashers and other "magic" stuff). Most of those are fairly simple and I won't really discuss them now.
LED flashers are kind of special. Most LED lights on the market have a built in flasher. This allows the light to blink when power is applied. Most also have multiple (sometimes tons of) patterns available.
Internal flashers may also have
a synchronization wire or other access point. This generally allows lights from the same manufacturer to flash together or opposite eachother. The capacity of this feature may be limited, only able to sync 2 lights total, or extreme, I remember a video from Feniex where they had a wall of T-6s syncd with eachother.
External flashers share many characteristics with internal flashers however they almost always require an independent power supply. The main advantages to external flashers is the ability to sync lights of various brands and a centralized wiring location. One of the main drawbacks of them is that not all lights will function properly. Some LEDs have a slight delay that we don't notice but it's enough to mess up the flash pattern from an external flasher. This is, in my experience, more common with lights set to steady that have an internal flasher. Most of the time your safest bet with an external flasher is to use "dumb" light heads.