University requirements might be different but most of them ask for recommendation letters. Leaving everything else (Degree, GPA, publications) they rely heavily on the recommendation letters. Not one but three recommendation letters. At least in my field (Life sciences) it's a well known fact that you have to be recommended as if you were the next "whatever genius you like". Since the PhD positions are limited because of many reasons (from "money" to the fact that "some top universities are proud of having a 5% acceptance") students must apply to at least (say) 10 different universities/institutes to have a minimum chance. Of course, they can be realistic about their chances and apply for the less crowded opportunities. That would mean making a list of your top 20 places and applying for the 10-20. After all, if they were "the next genius" they wouldn’t have any problem with recommendation letters. So far, so good, but they still need those letters. As far as I know, a "normal" pre-PhD person might have at most 2 different research experiences so if they are able to get one professor to write something good about them they're OK. However, this means that these 3 persons have to be willing to write 10 letters for you (and logging in the online system which asks them a lot of questions about them and about the student). Basically, they can:
1) Do copy-paste and change the name with a generic great letter. This means your recommendation letter won't be about you (or will be as yours as anybody else's)
2) Write a great unique letter
Usually a PI would have tons of students asking for recommendation letters so it's a natural part of their work to do some writing but it's still a big favor. If a PI writes too many outstanding letters the system will suspect He's a fool or even worse a liar. So my guess is that they just write "great" letters.
But, how is a great letter composed? Are the writers really aware of what they have to write? is the same letter suitable for two PhD programs? What if they're in different countries (I've been reading that south American and European PIs write "too realistic" letters for US PhDs and US PIs write "too good to be true" letters for European standards)? Then, should a pre-PhD student work in both continents first and then apply? How to deal with the fact that the PIs may be unknown? How to deal with the fact that you need many different PIs writing you many different letters? How to ask your PI a recommendation letter to "leave" him/her?
I know that it's not ONE question but for me it's part of the same problem.