Depending on the country in which you apply for it .... the rules are different. I did not need such a letter for my (Australian) application, which was an entirely personal application, not something my company could apply for. However, when a Taiwanese employee of mine tried to apply, she discovered the rules were quite different for her.
Come to think of it, that does makes sense and is in alignment with visa requirements for nationals of various countries.
You apply yourself; it's issued in your name.
Well, yes of course it is issued in your name, but in addition to the form, copies of these supporting documents are required:
- a photocopy of your Hong Kong permanent identity card (only the side with your personal particulars and photograph is required);
- a photocopy of your valid passport showing your personal particulars, nationality, dates of issue and expiry;
- another identical photograph of yourself; and
- a letter from your employing company supporting your application.
As noted by Movingin07, the rules are different depending on which country you apply for it in. In Hong Kong you need to be a PR to apply, so if you are an Australian citizen, non-PR in HK then you would need to apply in Australia, and there you (apparently) don't need any supporting documents from your employer.
Those are HK regulations. I applied for one based on my NZ passport and didnt need anything from my employer.