Along with one-way functions, algorithms for generating random-like numbers are considered cryptographic “primitives,” i.e., fundamental building blocks necessary for securing communications. Why is that? Simply put, any secret communication system depends on values that are known to the securing party, and unknown to the outside. That depends on generating random numbers. (Often, user supplied input is also incorporated, but humans are notoriously unreliable in providing such input.)
Most types of random seeding (like pulling from the system clock) are only “sort of” random. If a hacker knows how the numbers are generated, they could devise random number attacks. The ingenuity of hackers seems to keep pace with that of white hat coders. Indeed, the story of cryptography is one of back and forth jockeying, a kind of call and response of innovation.
More secure means of generating random numbers have been developed to foil random number attacks. Most crypto libraries include facilities for these generators.