It does give a reason, but can sometimes be an incomplete one, like 'old age' which can cover anything from a pulmonary embolism to a heart attack. I had a part time job as an organist for a local undertaker, and some of the paperwork from the hospital gave the impression of being written by someone wanting to clear off quickly for the night.
Apparently it is not possible, though I am not an expert, to tell how somebody died without a post mortem, so a fair chunk of the papers produced are based on incomplete information.