Homework is argued about as a single practice when it serves different purposes at different ages. Most disagreement comes from applying evidence about older students to younger ones.
In primary years the effect is small
Research on homework in the early years of school consistently finds a weak relationship with attainment, and the finding has been stable for a long time.
Young children have limited capacity for independent study, and the tasks that can be set unsupervised are necessarily simple, which limits how much they can add.
Where primary homework has value, it tends to be in establishing a habit and giving parents visibility into what is being taught, rather than in the content itself.
Retrieval and spacing explain the secondary effect
Older students benefit more, and the mechanism is well described. Recalling material from memory strengthens it more than rereading does.
Spreading that recall across days rather than concentrating it produces more durable retention, and homework distributes practice across the week almost by accident.
This is why short, frequent tasks that require retrieval tend to outperform long tasks completed the night before, even where the total time is similar.
Not all homework does the same work
Practising something already understood consolidates it. Attempting something never taught produces frustration and, frequently, a parent teaching it incorrectly.
Extended projects done at home mostly measure household resources: a quiet space, materials, internet access and an adult with time and confidence in the subject.
That resource dependence is the main equity objection to homework, and it is more pronounced for projects than for routine practice tasks.
Volume has a ceiling
The relationship between time spent and benefit flattens and then reverses. Beyond a certain point, additional homework displaces sleep, activity and family time.
The point at which this happens rises with age, which is why guidance usually scales recommended time by year group rather than setting one figure.
Fatigue also degrades the quality of the practice, so extra hours late at night can produce less learning than fewer hours earlier would have.
The useful household position
Consistency of time and place does more than supervision of content, because the main obstacle for most students is starting rather than understanding.
Parents explaining subject content can help and can also conflict with how the material was taught, which is worth checking before correcting a method.
Where homework routinely takes far longer than the school expects, that is information the school needs, since it usually indicates a difficulty rather than a work ethic problem.