Full Idea
By repeated transformations using the Deduction Theorem, any proof from assumptions can be transformed into a fully conditionalized proof, which is then an axiomatic proof.
Gist of Idea
Proof by Assumptions can always be reduced to Proof by Axioms, using the Deduction Theorem
Source
David Bostock (Intermediate Logic [1997], 5.6)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.223
A Reaction
Since proof using assumptions is perhaps the most standard proof system (e.g. used in Lemmon, for many years the standard book at Oxford University), the Deduction Theorem is crucial for giving it solid foundations.