Short answer: offload apps on iPhone when you want to remove the app itself but keep the related data for later. It is the safer option when the app is heavy, rarely used, and still contains information or setup you do not want to rebuild from scratch.
This matters because deleting an app and offloading an app are not the same thing. Offloading is usually the better storage tactic when you want space back without committing to a permanent reset of the app experience.
When offloading is the right move
The app is large but not something you use every week.
You want the option to reinstall it without starting over completely.
The phone needs space, but media cleanup is not the only available win.
When offloading is not enough
The real storage problem is photos, videos, screenshots, or chat attachments.
The app itself is small and the library clutter is much larger.
You need a bigger immediate space win than one or two dormant apps can provide.
How offloading fits into iPhone cleanup
Offloading is best used as one tactic inside a broader iPhone cleanup order. It helps when app weight is real, but it should not distract from the bigger categories if media and downloads are doing most of the damage.
If you need the bigger route next, open free up iPhone space. If you want to diagnose the weight first, continue to What Is Taking Up Space on My iPhone?.
Offloading is the safer app cleanup because it trades app weight for space without forcing a full delete-first decision.