IA essentially is in charge of ensuring that all operations across the company have set processes and controls for each process. They ensure that there is a solid segregation of duties for all these processes. They then are in charge of walking through these processes and testing our internal controls around these processes and ensuring that they are effective.
The auditors expect that IA's walkthroughs and control testing would be enough to rely on a sample of them. They also want IA to know all of our walkthroughs and controls inside-out, because that, in theory, is their job.
In the instances that I've been exposed to, both in audit and in private, it's really not feasible for IA to know these processes inside out. Most IA groups aren't really that big, so from a process perspective,for them to know all the nuts and bolts of a company would be hard (unless the IA group is really built out to handle or the company is really small).
If you ask someone in IA, their job is more than just reviewing processes, and testing controls. It's about various legal dealings, company structures, feasibility studies, etc.
One thing I can tell you - they don't work late, and there's decent travel involved if the company has various locations. Also, while they aren't despised like the external auditors, we're still not picking up the phone in a hurry every time they call.