Importance of features
I'm not sure irrelevant is the right word, but here is an example where it may not be an easy trend.

You're trying to predict blood pressure.

If you're only given weight, there may be some correlation, but someone who is 150lbs could be 5'10 or 5'4, and that would make a large difference to their overall health.

If you have both, you'll likely have a much stronger correlation to work with.

But as all things in ML you should test it yourself usingany real world examples as theory doesn't mean much if it doesn't hold true with real data.