HR doesnt (or shouldnt) really play a role in this... HR is a support function.. the Hiring Manager sets the standard for the person he wants to hire.. depending on the company, HR can have a constricted, or an expanded role.. and may participate in the recruiting function.. and may be the first "gate" to get past with your resume.. but at the end of the day, an HR rep is not the subject matter expert on all roles/responsibilities/duties in the company... they cant possibly know everything it takes to be a successful janitor.. as well as a successful project manager.. as well as a successful director of finance and accounting.. as well as a successful IT security manager.. as well as a successful widget engineer... etc..etc..
Hiring managers set hiring standards.. HR simply performs a role in the hiring process and helps (or hinders) your ability to actually get to the hiring manager to have your resume reviewed or get an actual interview..
Any manager worth his salt isnt going to let someone in another department... ESPECIALLY HR.. dictate to him what certifications, education, experience, etc.. someone needs to have to get hired.. At the end of the day, the hiring manager is going to be held responsible for the performance of both the individual and the department/unit/section/whatever that the person is hired into..
Different hiring managers will have different reasons for requiring a degree.. or not..
for many.. its a simple discriminator.. a degree isnt really required to perform the actual work.. but it tells the manager all of the things already listed above by others... right now its a buyers market... a degree (or lack of one) can act as a filter and help me bring the stack of 25 applicants down to 12 for review... other discriminators might help me further reduce that stack to 4 or 5.. once I get the stack whittled down that low.. I can start to make decisions about who to bring in for interviews..
for others the degree is an absolute requirement.. you arent going to hire a "doctor" that doesnt have an MD.. or a senior research physicist on a nuclear project without a PhD.. or an attorney without a JD..etc.. you also arent going to hire a guy to design a bridge that doesnt have a PE.. which requires a degree in engineering to sit for... etc..
For others still.. the degree may not be seen as a requirement to do the job.. but their customers expect or demand it.. so they make it a hiring standard.. this for example is a HUGE deal for companies that deal with the federal government.. the fed govt wont award certain contracts unless the people assigned have degrees.. just this past week for example I put together a bid for a contract that is going to be managed by the DoD.. the work is IT focused and calls for several very high end skills, certifications, and qualifications.. the IT guys that have posted in this thread are right.. in the IT world degrees tend not to matter nearly as much as certifications and experience.. this isnt typically true when dealing with the fed gov though.. the government was very clear on the work I am speaking to above.. every single person assigned to the contract has to hold a bachelor degree (they could care less what field it is in.. you just have to have one).. hold a security clearance (even though the contract is not classified and involved no cleared work).. and a laundry list of certifications..
In a nutshell, the government knows the certifications are whats important for being able to actually understand and accomplish the work.. but they are using the security clearance and degree requirements as discriminators.. if you have a clearance they know you have passed a credit check, a criminal background check, etc..etc.. its no guarantee that youre not a dirtbag.. but its a guarantee youre not a felon and likely arent in a financial situation where you would be distracted from work, etc.. if you have the degree they know you are likely a little bit older (probably took you a few years to get the degree before or after you started stacking up certifications).. hopefully are a little more mature.. etc...
so they force those standards on their vendors.. in order to get what they are looking for/want (which is often not necessarily what is best for them)..
Ive seen this in the private sector as well.. where large companies will push down standards to the companies that provide them services and products that force them to add in requirements to their hiring standards..... the vendor companies dont HAVE to meet the standard.. but the company requiring services/products dont have to buy from them either.. so the choice ends up being 1) walk away from the work/opportunity.. or 2) hire the type of people they want us to that will be handling their product/service..