The difference between the two is, the astable can perform only one function. . . to alternate between on and off at a rate determined by the resistors and capacitors.
A 555 can do this, but can do other things too, e.g. wait for a specified time, then turn on. Or turn on for a specified time, then turn off. Or etc etc
Basically, the 555 is more versatile (than the astable transistor circuit), and often just as cheap, or cheaper than using transistors to perform the same task.