Why using 555 timer in ir transmitter?

The 555 is a timer and oscillator chip capable of sourcing and sinking 200 milliamperes, and is useful for pulsing an infrared LED to power an infrared transmitter. It can be pulse width modulated or otherwise controlled via the various control pins.

See: http://talkingelectronics.com/FreeProjects/555/555-P3.html

in most cases the IR detector used is TSOP sensors which works only on a specified frequency of 38KHz. So by using 555 timer, we can design the circuit very easly to work @ 38KHz. dats why in 555 is used in many IR txns.