A computer systems engineer is a person who works on computers like fixing and upgrading them.(Sokanku)
They also give advice to clients on how to use their computers to their fullest potential.(Sokanku)
Sometimes they work in highly sanitized places that has a lot of delicate computer components, others work by going around and helping people.
They also design personal computers, systems networks and other things that utilizes a computer system.(Sokanku)
Computer science is used all the time in this career such as when understanding what kind of problem(s) a computer has and how to fix it.
It is also used in knowledge when an engineer recommends what kind of computer a client should use that suits their needs.
It other words, a computer systems engineer is a computer generalist who is good at multiple topics of computers.