I'd suggest two things are of value in this app: firstly, the the "format" of these particular editions, and, secondly, their curation as an app.
If you want to read "Excursions in Madeira and Porto Santo", by T. Edward Bowdich, Esq. (one of the books), then archive.org has an excellent copy, here:
However, you probably would never read such a book. You may, however, browse through the version included with the British Library app. The BL version is a very nice edition, with colour plates. More importantly, the BL version is presented as part of a collection of interesting old books, which means that a lot more people will be reading about Mr. Bowdich, Esq.'s adventures this weekend than would normally be doing so.
The presentation of the books is of general interest. By contrast, the actual text content of the book is of very specialised interest, and the (decontextualised) pictures not much less so.
The problem is that there are now a pile more mediums to deliver the message on depending on how much you can afford to pay out.
The information should be uniformly available to all, which I believe is the point of the British Library.
However, having been there a number of times, it's a pain in the arse to use the BL anyway and copying anything for reference (like you used to do down your local library) is virtually impossible now. So it's literally sit and copy the work out.
format is important in order to make sense of content.
I'm sure right now they have a huge database with lots of scanned images, and an app like this helps to make sense of it. If I'd have a big zip file containing all those books, it would be difficult to browse through them. They said they're planning to add support for other devices, so you may still get what you want one day.
Text files take away the writer's intent. "To Sherlock Holmes, she is always be the woman." That is a rare instance of Conan Doyle using italics. And he meant it.
What people don't realise is that the content is important and the format is not.