Imagined or not, there is/was a notion that greeting people helps for loss-prevention purposes. (i.e. if you say hello and make eye contact with someone as they enter your establishment, they are less inclined to steal from you.)

If Walmart is removing those positions, they may have found it to be a misconception. [edit: it seems like Walmart is not removing greeters, just making it harder for disabled/elderly people to hold those positions and expanding their responsibilities.]

They're also calling them hosts, which is annoying, because they aren't anything like a host at a restaurant, because going shopping in a giant big box retailer isn't anything like dining at a sit-down restaurant. I can see why they would use an overblown title, because they call their employees associates to say that they consider them partners, when in reality there are over a million employees and there's no way they could all be partners.

They seem to be changing the positions from being generally at the door to instead being staff generally designated for welcoming but also helping people with requests, in which case 'host' makes sense as a distinction from other staff intended to focus on stocking, cleaning, checkout, etc.