What does it mean to be offered waived-fee OA publishing?
It is still an open question whether open access actually leads to more citations or not.  This is at least partly because the models and players in OA are still rapidly shifting, especially with the rapidly increasing influx of low quality or scam publishers who have embraced open access because it is an easier way to bilk money out of insecure early-career academics.  

I will assume, however, that you have already vetted the journal and know that it is a significant and reputable one in your field.  Let us also assume that it really is just as you present: the article will still be published in exactly the same way, just not behind a paywall and this will cost you nothing.  In that case, the general wisdom is that, all else being equal, an article that's easier to find and read is likely to have higher impact than one hidden behind a paywall.

Free open access in a normally closed journal is actually less unusual than you might think.  For example:


Nature makes certain types of large community studies open by default.
Special issues and special collections in a journal are sometimes negotiated to be open access by their organizers.  If your article fits with such an upcoming collection, it would be natural for it to become included in the policy.
Certain types of information, particularly in biomedical research, are required to be published open access (either immediately or within certain timeframes) by some government funding agencies


The real question is: what is the reason in your particular case?  Ask the editor. If it's something that makes sense, like one of the cases above, then great!  Embrace it and get some extra citations.  If not, then ask yourself again whether you really trust this journal...