Full Idea
You can establish facts of the form Γ|-φ while avoiding the agonies of axiomatic proofs by reasoning directly about models to conclusions about semantic consequence, and then citing completeness.
Gist of Idea
In a complete logic you can avoid axiomatic proofs, by using models to show consequences
Source
Theodore Sider (Logic for Philosophy [2010], 4.5)
Book Reference
Sider,Theodore: 'Logic for Philosophy' [OUP 2010], p.105
A Reaction
You cite completeness by saying that anything which you have shown to be a semantic consequence must therefore be provable (in some way).