"Abstract" fields can be embedded (somehow) into the field of complex numbers. So what are the advantages of considering these abstract fields?
It is said that prime ideals in rings are "behave" like the primes in the ring of integers. A number field contains therefore two objects that are like the prime numbers, namely the prime numbers itsself since it is a field extension of the rational, and the set of prime ideals. This is a little bit strange...

