Halogen flashers will flash LEDs cuz they're made to work with halogens which use more power vs LEDs which require a lot less.
Be careful! This is not always the case. Some halogen flashers require a load of an amp or two in order to flash.
For instance, there are two versions of the Federal Signal FA3 flasher. The original version has the same three terminals as a 537 thermal flasher: +12 vDC in, +12v DC out to load A, and +12v DC out to load B. However, the newer version of the FA3 has a ground wire in addition to those same three terminals. With the ground wire connected, the FA3 will run regardless of whether or not a load is present. This allows it to flash LEDs. The older version of the FA3 does not work well with LEDs, or with small loads in general.