The purpose of HR is to protect the company. Hiring managers tend to ask the wrong questions and get the company sued. They also tend not to care about hiring quotas which exist at most big companies.
I can make the same argument for any function in regards to protecting the company - yours is related only to HR regulations/requirements. As an example: try the medical device world where the HR people (and others) want to approve suppliers of services outside the process or can’t document training to save their lives.
The issue is that in order to move a company forward there must be constructive conflict: goals, objectives, competition. Could you ask the appropriate questions related to statistical analysis, strength of materials, CFR 820, or ISO 13485 - likely not, nor would I expect to negotiate the world of college job fairs, ADP payroll, required postings, or insurance benefits.
A hiring manager knows what skillset they need technically to overcome said conflict and in some cases they cannot find it because the good ones are screened out too soon.
Quotas (if reflecting race, creed, gender), imhop, are imposed illegal bigotry and have no impact on if the job can be accomplished by the candidate. When they are established they also typically include a statement that the individual is STILL required to meet the minimum requirements. I’ve often been forced to sit through interviews with unqualified candidates because HR didn’t want to take the heat for turning them down. (If your referring to headcount that’s an issue of budget for which the manager should already be aware)
The hiring manager has to deal with meeting the Gs&Os with the resources available and have final say for their staff everywhere I have been and rightly so.
HR has a purpose - but sometimes they, as with all functions, overstep and try to dictate decision results instead of procedures and process. As with all functions I’ve worked with good and bad.