<p>I am working on a C++ project and I noticed that we have a number of warnings about unused parameters.</p>

<p>What effect could it have if these warnings are ignored?</p>

None. except [EDIT]: as others have pointed out, You could have an unassigned output parameter.
You should clean them up, because on many occasions I have seen developers ignore important warnings that were 'hidden' in amongst a large number of warnings, and they were so used to seeing warnings they never paid any attention to them. I try to have zero warnings at all times, and set compiler warnings to the maximum level.