I've just never had the option with a corporate job so I'm not used to it. I think the assumption with businesses is that they don't have the time or resource to show 100 candidates around the office before they've even applied!
Well, I'm not NHS, but schools generally welcome visits - it saves applicants time in that they don't have to waste time applying if the school isn't for them. It therefore saves the schools time wading through application form from people who might then turn it down if offered. It means - by not shortlisting people who later realise it's not for them - you have room on your shortlist for people who really know what they are getting into, and are committed to thinking they can do a good job. The time given pays for itself in time saved.