Creasers are very simple but somehow also very imperfect machines that are necessary due to how most binders work. They're for creasing covers, to mark where to bind, and to create a cleaner spine, and sometimes (secondary creases) to make the cover open more easily. We only have experience with manual creasers. They do not vary much, just look for sturdy-looking and -feeling construction.
Alternatively, if you have access to a powered letterpress like a Chandler & Price, it is possible to set that up for creasing a large run in a short amount of time.