Integrated circuits were one of the biggest break-throughs in the history of electronics. Being far smaller than the traditional discrete circuits meant they could be included in a huge range of electronic equipment at a lower price, and as if that wasn't enough, they have better performance too.
The cost is kept low by the manufacturing process of integrated circuits. Essentially, they are a cluster of electronic circuits on a single, tiny plate of some semiconducting material, most commonly, silicon (thus the name 'Silicon Valley for the world's programming capital). This 'chip' can be assembled in its individual components by a photo-lithograph that 'prints' it as opposed to being assembled one transistor at a time, as was the previous case. Being small, there is also the significant benefit that it takes far less materials to construct a package of integrated circuits, as compared to the now-defunct discrete ones.
All of this is owed to experiments during the 20th century that showed semiconducting devices could perform the same functions as the vacuum tubes which early computers relied on. From then, the discoveries poured in. Advancements in the manufacturing of semiconductor devices, and the idea of integrating huge numbers of transistors into a tiny chip far outstripped the benefits of manually assembling the circuits using discrete electronic parts. Today, integrated circuit devices exist that have upwards of several billion transistors in a chip no larger than a human fingernail.