Why does a copy editor count the number of characters on a line? I got my paper back with remarks of the copy editor. One annotation counts the number of characters in a line. I have been told to ignore it, but I am still interested to know what it is for.

On the bottom of the page, the copy editor wrote: "73 × 39 × 27 &rarr; 25p.". 73 is the number of characters, 27 is the number of pages of my manuscript. 39 is the number of lines per page minus 1. It seems to be a calculation for the final number of pages (judging from the "p.") but I don't see how that figure would be calculated by multiplying these values.

I am using a template provided by the journal.
73 x 39 x 27 would give an estimate of the total number of characters in your manuscript.  They presumably have some standard estimate for the number of characters that fit on one page in the final format, and they've divided by that number (not shown) to get 25 pages.

I can't think of a good reason to subtract 1 from the number of lines per page (unless one line is obviously a header or something) so I'd wonder if they just miscounted.