Unlike whether you need an air-filled memory foam rachet back height adjustment, however, the problem is not clarifying the benefit or explaining a complicated feature of a chair. The issue that makes this question difficult is the simplicity. It’s hard to feel like I’m being of much help when my only response is “Would you like somewhere to rest your head?”
But, the simple fact is that a headrest is a personal preference call for most people. The posture most people use while working does not put their head in contact with the headrest, so it is primarily there for times when you wish to lean back and take a quick rest from holding yourself erect. This can be a benefit, but it is not one everyone feels they need. Some really do like them, though, and will not do without.
If you are a headrest aficionado, I can recommend a few chairs with particularly good ones, though.
ErgoCentric chairs like the GeoCentric have optional headrests that adjust in and out as well as side to side. This is a fairly unique feature on a chair in that price point. Nightingale CXO chairs share that trait and like the rest of the CXO, the headrest feels wonderful. The Freedom Chair is one people rarely order without a headrest, with good reason, in my opinion. The headrest on that chair is well aligned with the rest of its unique recline mechanism to provide support at the points where you need it.
While the question of whether you need a headrest at all is largely one of personal preference rather than ergonomic need, we do offer several chairs with great headrests for those who want them.