DS has asthma so respiratory illnesses often hit him harder than the rest of the family.
A few years back he had a few days off for a cold that became a chest infection. His asthma later flared resulting in a 999 call from school, and being unable to leave the house for a week until the course of steroids took effect.
He then had an emergency surgical issue requiring more time off.
This triggered The Letters. Pretty shitty when you've had the stress of a child with serious health issues requiring two A&E visits in as many months.
Later in the year he had a random sick bug... 48hr rule and all that. Because he'd had a certain level of absence, school would no longer authorise illness without medical evidence. I am not taking a child to the GP for a bit of vomiting. It was marked unauthorised which I didn't realise until the end of the year to challenge it. This does matter in that this can be used to fine families if "unauthorised" absences stack up which can happen easily in children with chronic health problems.
I don't send my children in ill for the sake of it, nor do I keep them off because they sniffed. I look at are they in a state where they can learn. I've often had one child on 100% and the other in the "persistant absence" zone, not helped by illness season being weighted in the first half of the year.
Absence does affect educational outcomes, but the current government strategy and blanket school policies really don't help the situation, and support parents at what is a stressful time in itself. That's on top of workplace pressures. If it's a choice of sending an ailing child in versus legal threats, I can see why parents are unwilling to test the legal threats route.