- the transactional part of setting up interview loops, coordinating schedules, etc is extremely tedious for a field of 3-5 serious candidates for a given role. Internal recruiter takes all of this away from a hiring manager who has other duties.
- an internal recruiter can plow through LinkedIn to find solid candidates, identifying ones for the hiring manager to review and, if qualified, reach out to. Again, tedious work for a hiring manager with other responsibilities.
Your job should also include lots of time on LinkedIn searching, as many HMs do today. Letting a recruiter do this for you will give you many more bad interviews (expensive) than you'd get otherwise.
But apparently "tedious work" is above a HM. Have fun being on conference calls all day, pretending that you and not the engineers are the ones who do the work. Worker self management needs to become more of a thing in this field in general if we can't trust HMs to take an active role in candidate selection. Companies that disvow managers and empower their employees produce far more excellent work (e.g. valve) even if they don't always do it on the schedule that execs want - so this would be a net positive for the industry.
In most cases that I've seen the internal recruiters have strong relationships with the hiring managers and are incentivized to make sure the hires actually succeed.
The only roles I've ever accepted were roles that involved a hiring manager reaching out directly to me. There's a reason for this, namely that the only recruiters worth a damn seem to be working in boutique firms and want to take a significant bite out of my future pay.
Really, just avoid them if possible.