What is the 'accumulator' in HQ9+?

I was just reading a bit about the HQ9+ programming language:

• https://esolangs.org/wiki/HQ9+,
• https://en.wikipedia.org/wiki/HQ9+, and
• https://cliffle.com/esoterica/hq9plus,

and it tells me something about a so-called “accumulator” which can be incremented, but not accessed. Also, using + doesn't manipulate the result, so that the input

H+H

gives the result:

Hello World
Hello World

Can anyone explain me how this works, what it does, and whether it makes any sense? Thanks.