We're talking here, of course, about a period for which essentially no written records exist – so the only way of trying to understand what was happening, and what the iron age peoples who lived near bogs understood of their properties, is to extrapolate from the available evidence. Nonetheless, enough bodies, and sufficient contextual material, survives to allow us to be able to address your question.

To begin with, it's probably worth noting that the idea that all the bodies found in bogs were human sacrifices is a little bit outdated now, and indeed archaeologists who specialise in this field remain divided on whether or not "sacrifice" is a suitable word to describe any of the remains that have been found, even the bodies that bear evidence of having been done to death in ways that Miranda Aldhouse-Green (one of the specialists who actually is happy to write in these sorts of terms) characterises as "ultraviolence" – by which she means people who sustained a wide enough variety of injuries to have died several times over.

Wherever one chooses to stand on this issue, though, I think that one key point is that at least some of the remains found in bogs seem to have been despited there by accident – the body known as Uchter Moor Girl, for instance, bears no sign of injuries and it has been suggested she slipped and fell into the bog while hunting for birds' eggs and bilberries. What this seems to suggest is that these bogs were not forbidden places, used only intermittently for the most solemn purposes, but rather resources that the people who lived nearby exploited on a much more regular basis.

There is plenty of evidence to suggest that this was the case. For one thing, we know that bogs were the principle source of iron for many of the peoples of this period; for another, bogs were apparently also used to store butter, as the several hundred finds of wooden casks that had been placed in bogs and which contained anything up to 100lbs of the stuff suggest.

What all this implies is that many of these places were visited frequently enough for the sort of accident that seems to have befallen the girl who died at Uchter Moor to have happened relatively frequently – and, hence, for sites in which bodies could be seen preserved beneath the surface of the water to have been known.

The preservative qualities of alkaline bogs, then, very probably were understood by the peoples who deliberately deposited bodies there, and it's certainly possible that those properties were one of the reasons for selecting them as places of deposit in the first place. While we can't claim to know enough about iron age religious practices to know for certain what was intended when bodies were placed in bogs, sufficient evidence survives – in the form of what appear to be the remains of stakes and hurdles – to suggest that at least some of the remains we have uncovered were very deliberately sited, and secured in ways intended to ensure that they remained in place.

I wrote about the topics of bog bodies, bog butter, bog iron, bog guardians and finds of "stakes" and "hurdles" much more fully in an essay you can read here, and this looks in much more detail at the evidence that interests you, as well as offering a fairly extensive bibliography.