A journey includes both setting out and returning. Someone returning from an expedition must also attend to life and family responsibilities. The value of the return is understood in the expedition's particular context.
The point of a simile is to clarify one thing through comparison with another familiar thing. In قَفْلَةٌ كَغَزْوَةٍ, the two are not identical in every respect; the expression highlights a relevant likeness.
Example: a learner translates قَفْلَةٌ as “a return” and claims that every ordinary commute carries the expedition's specific significance. That misses the context provided by غَزْوَةٌ. Read the words together before drawing conclusions.