Full Idea
The method of natural deduction is popular in introductory textbooks since it allows reasoning with assumptions.
Gist of Idea
Natural deduction helpfully allows reasoning with assumptions
Source
Theodore Sider (Logic for Philosophy [2010], 2.5)
Book Reference
Sider,Theodore: 'Logic for Philosophy' [OUP 2010], p.37
A Reaction
Reasoning with assumptions is generally easier, rather than being narrowly confined to a few tricky axioms, You gradually show that an inference holds whatever the assumption was, and so end up with the same result.