Sometimes you’re asked to finger a chord in somewhat cramped quarters — especially up the fretboard a bit — or you’ll want to be adding extra notes to a chord (like for a “chord solo” type arrangement). A good trick to learn is how to use one finger for more than one string. This also can help to more speedily grab a chord position.
Note that I’m not necessarily talking about the “classic” barre chords here, this is using your fingertip (actually, more like the last joint of your finger).
In the picture to the left you can see a close-up of me fingering a Dm chord (2210 in GCEA tuning). Note how my middle finger has rolled back a bit from the tip, covering both the G and C string at the second fret.
This leaves my pinkie and ring fingers free to move around on the melody notes.