I would think that any nice, lengthy journal would be cake to a historian. An old curator of mine has been obsessively studying a journal that, on the surface, details a very ordinary life. But it's full of details about textile work that the woman was doing - sewing, knitting, weaving, processing fiber - and it's of immense use to textile historians. The woman writing it probably had no idea that her daily chores and interests would fascinate another woman so many years later.

I've never been able to keep a paper journal going, but internet ones have been more successful for me. Although I do keep a booklog which is updated very infrequently but which allowed me to use a blank book I would otherwise hoard. I adore beautiful blank journals, but then never have anything to put in them!