Calling a Random Number Generator (RNG) that is not seeded, will result in generating the same sequence of random numbers in different runs of the program.
Suppose there is a code that calls 10 times an RNG function to produce a sequence of 10 random numbers. Suppose, also, that this RNG is not seeded. Running the code for the first time will produce the sequence S = <r1, r2, r3, r4, r5, r6, r7, r8, r9, r10>. Running the code again for a second time will produce the exact same sequence S. Generally, any subsequent runs of the code will genarate the same sequence S.
As a result, an Knowing the sequence of random numbers that will be generated before hand can lead to many vulnerabilities, especially when security protocols are concerned.
Calling rand() function several times to produce a sequence of pseudorandom numbers generates the same sequence in different runs of the program.
...