What kind of post you are actually setting
A post in the middle of a straight fence run mostly resists being pushed sideways a little by wind, livestock leaning on the wire, or the fence settling over time. Tamped soil or crushed stone around that post, packed in firmly in layers as it goes back into the hole, handles that load fine on most ground. That is why you see miles of pasture fence on tamped posts with no concrete anywhere on the line.
The load changes at a corner, an end, or a gate. Those posts anchor the tension of the whole wire run pulling in one direction, or carry a gate swinging open and shut thousands of times over its life. That is a sideways and twisting load a straight-line post never has to deal with.
The fence material matters here too. A light electric line or a single strand of wire barely tugs at a post either way. Woven wire pulled tight, field fence, or a heavy livestock panel run puts real, constant tension into every post it is fastened to, corner and straight-line posts alike, which is part of why the same fence can call for concrete in some spots and not others.
Why corner and gate posts get concrete more often
Concrete around a post resists exactly the kind of pull a corner or a gate puts on it, because it turns the post and the soil around it into one stiffer unit instead of a post that can rock loose in its own hole over time. A gate post especially takes repeated leverage every time the gate swings, which works a tamped post looser year over year in a way a straight-line post never feels.
That does not mean every corner needs a poured footer. A light wire fence with a small, low gate puts far less load on its posts than a heavy livestock gate or a long fence line pulled tight. Match the post setting to what it actually has to hold, not to a blanket rule.
What the ground is doing under the post
Soil matters as much as the load. Loose, sandy, or consistently wet ground gives a tamped post less to grip, and it can work loose faster than the same post set in firm clay. Frost heave, the ground swelling and settling as it freezes and thaws each winter, moves a post that is not set deep enough or braced well, and a few winters of that can leave a post leaning even if nothing ever pushed on it directly.
This is the kind of ground condition worth thinking about post by post rather than for the whole fence line at once. A corner sitting in a low, wet spot of the pasture is a different question than the same corner on a dry rise fifty yards away.
Where concrete helps and where it does not
Concrete anchors a post against sideways pull and against ground movement, which is exactly what a corner, an end, or a gate needs. It is not a fix for a post that was never set deep enough to begin with, and it is more than a straight run of light fence on firm, well-drained ground usually needs.
If a post on your line is doing more work than holding up a wire, a corner carrying tension from two directions, or a gate post taking real weight and repeated leverage, that is worth a second look before it goes in the ground rather than after it starts leaning. Most fence lines end up using both approaches: tamped posts for the long straight runs and concrete at the handful of spots doing the heavy lifting.