I spent a productive half hour train journey a few weeks ago drawing a location plan of a house and garden on a headland somewhere in Wales. It is fictitious, hence the need for a plan. I'm trying to get the story straight in my mind and having done timelines I realised I needed a map as the location was a hole in my thoughts.
I'll also want a house plan and one of the garden, but I'll get to that. They are for me and I doubt if I'll ever share them anywhere, but it made me think of a discussion a little while ago in the Crime Writers' Association magazine, Red Herrings, on the value, or not, of plans, cast lists and family trees appended to books for illumination of readers. Some liked them, others didn't. All a matter of taste.
As a device used now, they sometimes have an old fashioned feel - I recall a number of Agatha Christie novels with floor plans as part of the text. - they would be an authentic touch for current cosy crime books that have an historical setting. I know I have found a cast list very useful when there are a lot of characters involved and a family tree can be invaluable to work out who is related to who - they usually appear in multi generational family sagas.
The big difficulty these days is, of course, reading electronically. If any or all of these reader aids appear in a book they will be at the beginning. In a paper book there is no problem in flipping back for a quick look - a bit more complicated keeping your place on a reading device.
Maybe this will mean that the practice eventually dies out? I think it would be a loss if it does. It doesn't suit every book, but if the author feels that it adds to the story, then why not?