Beneficiary, Purpose Or Qualifier?

The first interpretation assumes that the prepositional phrase can be thematised:

for postgraduate studies, we need to develop good academic writing skills

The second interpretation assumes that it cannot.

The interpretation of the prepositional phrase as Beneficiary (Client), is ruled out by the impossibility of omitting the preposition:

* we need to develop postgraduate studies good academic writing skills

Halliday & Matthiessen (2014: 237):

The two functions of Recipient and Client resemble one another in that both construe a benefactive role; they represent a participant that is benefitting from the performance of the process, in terms of either goods or services. The Recipient is one that goods are given to; the Client is one that services are done for. Either may appear with or without a preposition, depending on its position in the clause (gave John the parcel, gave the parcel to John); the preposition is to with Recipient, for with Client.

Postscript:

The indeterminacy in this (constructed) clause arises from the nature of the grammatical metaphor deployed. A more coherent rendering of the metaphor would be:

we need to improve the academic writing skills of students undertaking postgraduate studies

Note also the incoherence of

we need to develop good academic writing skills for (the benefit of) postgraduate students

where the postgraduate students are construed as benefiting from someone else's improved writing skills.