The main reason for digitising archives, according to the NA themselves, is in order to be able to preserve the old and fragile documents and other materials, which can often range several hundreds of years back. In this manner it is easier for several researchers to work with the material without that damaging physical touch. I remember being told, when we visited the National Archives with a group of uni students, that texts written with a pencil lasts much longer than any kind of new, modern kind ink (ball-point pen e.g.). Better keep that in mind while writing notes by hand :'3.
Personally, the National Library is a bit more familiar to me, but rather through its physical books than digital archives. They also have a smaller collection of digitised material, which, according to themselves consist of "Finnish newspapers, historical everyday ephemera and vintage audio recordings are among the materials digitised by the National Library of Finland for freely accessible online use". These collection consists of 'The Historical Newspaper Library 1771-1909, a very valuable resource indeed (have to take a better look at some point, I really like old news papers). Just like the NA, the NL of Finland digitises material and collections it considers culturally and historically valuable, as to facilitate the use of the materials and preserve the original publications. Much good it would do to 200 years old news papers, if every slightly interested student wanted to physically browse them.
On my opinion of digital archives, I already contemplated the subject in my earlier post, but to add to the topic more specifically; I don't reject or refuse digitisation of old archives - it's practical and efficient for many reasons. I confess I have not really given it any bigger thought than what I presented earlier, because I have yet to use it more frequently. In the end, I'm a romantic person, so maybe that's one of the reasons why I like books - they have that feel of nostalgia that clear, clinical digital publications don't have. In the end, I'm a lazy person, so despite my fidelity to good old books, I still like being able to access books and articles online without having to make a bigger effort. True story, bro!