Are online-only sister journals simply a way to generate revenue? Some journals have "sister" "open" journals which are online only and require article processing charges as opposed to their counterparts by the same name (e.g., JRSM/JRSM Open, BDJ/BDJ Open). Do they tend to have a lower standard for publication just to generate revenue? Or is it actually to promote scientific publishing?
Can it be both?  In the cases I am familiar with, these online-only journals serve primarily to publish marginal papers.  These are papers that are of fairly limited interest, but they still have some value, and it would be a shame if they were not published somewhere.  At the same time, they provide an additional revenue stream for the journal.  The net result is that the journal makes extra money and manages to publish more material, with limited additional costs to themselves.

I have experience with one of these journals, myself.  A co-author (who needed one more paper for tenure) picked the journal, which was advertised as considering only the correctness, not the importance, of work as a criteria for publication.  However, the referees were much harsher than they ought to have been, based on the journal's stated policies.  The referees took whether the work was significant quite seriously, even though, in theory, they should not.  So the result was quite a rigorous review process, which forced us to improve the paper.  The journal got a sum of money, and a worthwhile paper was added to the literature, so both goals were met.