Care homes are usually very hot, so there's sweat.
Being less mobile means that pressure can cause sores on the back of the head - which need to be checked (and preferably prevented or stopped before they deepen or get infected).
It reduces the likelihood of tangles developing into matts that have to be cut out or stressful brushing.
Older people can still get nits. They can also have skin conditions such as eczema and psoriasis, both of which can leave the scalp vulnerable to infection (from MRSA, for example), splitting and burning/itching sensations.
It soothes the scalp, preventing itching, which can drive anybody to distraction.
It involves close care, which enables the carer to observe any changes, perhaps in vision, whether facial muscles have dropped to indicate a stroke or TIA, neck strength, coordination and bruising (for any cause, including medical, accidental or even non-accidental ones).
It can be soothing and relaxing. It's gentle, positive and safe contact with another human being, warm water, shampoo and conditioner, an element of scalp massage
And residents are still entitled to dignity, which will mean being clean & fresh, which includes their hair.
Why wouldn't you want a vulnerable person to be given that level of care? The alternative is leaving them feeling itchy, looking dirty, perhaps with ratty hair or headlice, infection, never feeling a pleasurable touch, of nice scent and warm water and possibly something serious being missed.