The (1) in make(1) corresponds to which section of the manual[1] make is in. This is useful in some cases to distinguish between things that might be in multiple sections like printf(1) the user command and printf(3) the C library function. When everyone knows what's being discussed, I think it's mostly people trying to give a shibboleth that they've read the fine manual.
Unlike “ls” and “cp”, “make” is a real word, so to help us human readers parse the document’s prose, adding the section number in parentheses gives our brains a quick clue as to what’s going on.
Or just reflexively included just in case, because of commonly encountering potentially ambiguous situations that you just disambiguate by default, even for non-ambiguous situations.
I guess to avoid the confusion with the verb and make the sentence more pleasant to read. I think the more usual way to deal with that type of problem is to use italics.
As the previous replies stated, the "1" is the section of the manual the page you are requesting occurs in.
If you have trouble remembering what the sections are, I recommend running "man man" in the terminal. The section numbers are explained near the beginning.
Irix (IIRC) used to respond to the argumentless command "man" with the supremely snotty "appropo what", which made me laugh out loud the first time I saw it
