On their about page:
>OpenStreetMap has a list of (nearly) every bench in the world – but they don’t record whether there is an inscription on it.
Weird since OpenStreetMap does have an inscription=* key, which could be combined with the historic=memorial and memorial=bench tags. So what the site does could very well be achieved with OpenStreetMap alone.
I understood that to mean not that OSM can't record that information, just that it doesn't - I assume people tend not to include that level of detail when contributing to OSM. So the benefit of this site is that it focuses contributor's attention on the inscription and prompts them to include it.
It's a cool idea - I wonder if they feed the data back up to OSM. If not they (or someone) presumably could easily enough.
A general tendence in OSM is that things that "aren't mapped" will end up being mapped. For example, natural=tree was originally intended for notable trees only, but if you look at the map today you'll see each individual tree mapped on parks, boulevards, picnic sites, regardless of its notability.
So in OSM, it's not that it can't record, or that it doesn't record. It's always that not many mappers haven't yet focused on the topic.
I've personally added inscriptions to the Map because I noticed other people had. I assumed, therefore, the data was coming from OSM. Then I noticed they have pictures of the benches too.
There are lots of memorial benches around where I am. After reading your comment, I decided to go look at an example photo. Randomly zoomed in to Seville, Spain and click on one.
You can link to images in OSM too[1], but you need to find a place to actually host the files. So far the best option seems to be to upload the images to the Wikimedia commons and then link from OSM using the wikimedia_commons tag[2], though the UX of that is pretty awkward.
A long time ago, so the details are vague and may have changed, I was interested in adding trees and tagging their species in OSM. I asked some people that know more about OSM than me and was told that OSM doesn't want to store data that isn't used for rendering the maps.
There are sometimes links to external data in OSM using Wikidata for example.
Weird since OpenStreetMap does have an inscription=* key, which could be combined with the historic=memorial and memorial=bench tags. So what the site does could very well be achieved with OpenStreetMap alone.