Here's one way philosophy PhD admissions could go: Your program offers admissions to the N top-rated applicants, figuring that X% will accept. If the acceptance rate looks like it will be unexpectedly low, then you expand admissions to the N+M top-rated applicants. Financial support packages could be done in a similarly neat way.
Often, things aren't quite that neat.
One way that they can be less than neat involves a department's gauging the interest of an applicant before offering admission or financial support. Here's an experience I had as an applicant in the 1990s: A professor called me from one of the schools to which I'd been admitted, and he told me that they had only a few "top tier" financial support packages to offer to prospective graduate students. He said they would be happy to offer me one of those packages if I was likely to come, but they didn't want to waste it on me if I was likely to go somewhere else. I told him I hadn't ruled out his school yet, but that I had a greater level of initial interest in a couple of other schools. I did not receive that financial support package and decided not the visit the school.
That was a fine outcome for me. I'd kind of thought of the school as a "safety" school anyway. The professor correctly guessed that my application was strong enough that I'd been admitted to higher prestige programs than his. It would have been unusual for an applicant in my position to choose his school over those others.
Although that was a couple of decades ago, I think the practice isn't unusual. Recently the APA's Committee on the Status and Future of the Profession, of which I am a member, discussed an email from a former PhD applicant suggesting that the APA adopt a policy against departments' contacting applicants to gauge their level of interest before making offers. Apparently she had been contacted by the Director of Graduate Studies at a department to which she had applied but to which she hadn't yet been admitted. Her sense of the conversation was that the DGS was prepared to offer the student admission if the student committed in advance to accepting the offer. She felt that this constituted illegitimate pressure to decide about a school before the conventional April 15 deadline.
In general, it seems to be in the interest of the profession if applicants can see the full range of offers and then choose the offer that fits their interests best rather than being pressured into accepting early offers out of fear, possibly at schools that are relatively poor matches for them. (Here's the official APA statement on the April 15th deadline for accepting graduate student aid offers.)
I, and some other members of the Status and Future Committee, are interested in others' thoughts about this issue. The APA might be willing consider clarifying or revising the APA's April 15 deadline policy, if that seems desirable.
The official wording of the APA policy is that "Students are under no obligation to respond to offers of financial support prior to April 15". Situations of the sort described above don't appear to violate the letter of this policy, since support has not been formally offered. However, it could be argued that informal conditional offers violate the spirit.
On the other hand, we might use hiring as a model, and it's common in both academic and non-academic hiring for the hiring department to gauge applicant interest before making an offer. Also, practically speaking, some departments have "hard caps" on enrollment or funding so that they cannot make more than N offers for N slots. Departments with hard caps will be in a difficult situation if several candidates who are unlikely to accept wait until April 15 to decide. Other departments, even with softer caps, still might not be able to rely on higher-level administration to return the slots to them if applicants decline. Departments in either of these positions might understandably want to reserve some of their primary offers or waiting-list offers for applicants they think are likely to accept; and part of this process might involve informally asking applicants about their about likelihood of accepting an offer if one were to be made.
Discussion welcomed below.