Attention leaves whatever it is meant to be on with striking regularity. The pattern is universal enough that it is better understood as how the system operates than as a failure of concentration.
Wandering is an active state, not an absence
When a person is not engaged with an external task, a distinct set of brain regions becomes more active rather than less.
That network is associated with recalling the past, imagining the future and thinking about other people, which is a reasonable description of what a wandering mind does.
So the drift is not the brain idling. It is the brain switching to a different mode of processing that runs whenever the demands of the moment permit.
It appears to serve planning and consolidation
Much of what surfaces during undirected thought concerns unresolved matters: conversations to have, decisions pending, problems not yet solved.
This looks like a mechanism for rehearsing situations and integrating recent experience, which is why solutions frequently arrive during walks and showers rather than at a desk.
An entirely focused mind would lose that function, which is one reason attempting to eliminate wandering is the wrong objective.
The cost lies in what accompanies it
Research using repeated sampling of ordinary life has found that people report lower mood while their minds are wandering than while they are engaged, across most activities.
The effect is stronger when the content is unpleasant, but it appears even for neutral content, which suggests that the state itself contributes.
Wandering also precedes the mood rather than only following it in these samples, which is what makes the finding interesting rather than obvious.
Noticing the drift is a separate ability
How often the mind wanders and how quickly a person notices it are distinct, and only the second is practically useful.
Long stretches of undetected drift are what produce the experience of reading a page without absorbing it or arriving somewhere with no memory of the journey.
Most contemplative training targets the noticing rather than the wandering, which is why instructions emphasise returning attention rather than holding it.
Some tasks tolerate it and some do not
Routine, well-learned activities can proceed largely without supervision, which is why they are where wandering occurs most.
Tasks requiring comprehension or judgement degrade quickly when attention leaves, and the person often does not register the degradation at the time.
Matching the demand to the state, rather than expecting sustained focus regardless of task, is generally more workable than treating every lapse as something to correct.