A language is always changing. New words come, old words go. So it is not a question of what a new word contributes... … just like in fashion.
I modified your n-gram to include letzten Endes. The result is very interesting. You can see an even faster rise of letzten Endes followed by a steady decline. LetzendlichLetztendlich is just hip at the moment. Maybe because it is shorter. Maybe because it has no Genetive. Maybe because it spares people the second of doubt as to whether to write it with a capital or not. However, chances are that letztendlich will have its peak someday and then a new star will be born.
At first I though it could be letztlich... … but no... Google … Google n-gram shows that it has overstayed its welcome already. But effektiv is to be watched out for ;)