"The final major step in the development of the general purpose electronic computer was the idea of a stored program..." Brian RandellA significant breakthrough at Manchester in 1948 was the practical realisation of a store that was both reliable enough and large enough to contain a sequence of instructions - the program - as well as its data.
The immediately perceived benefit was that a program could alter itself during its execution. This is a technique that was a simple necessity in the days of such primitive hardware and the almost complete absence of software support. Today such self-modifying programs are rarely used.
The crucial point is that a universal machine can be specialised by simply loading a program into store.
Somewhat later it was realised that the concept of a stored program would open up countless opportunities. For example, in the years following Kilburn's first program, sophisticated program development aids were developed at Manchester and elsewhere. Now universally known as compilers, these are programs that translate from a human-oriented expression of a program, in a combination of algebra and English-like phrases, into the machine-oriented order code of a given computer.