I apologize for the big picture. My house was built in 1956 and has no grounding system although it has bootleg grounds on some outlets. If I were to plug something into the outlet and, somehow the outside of the thing of the thing had a conductive path to the narrower, hot, hole I would be at 120V relative to ground. This doesn’t cause a problem until you somehow ground yourself, then current will flow, which can kill you. A grounding prong, which cannot be plugged in there, would prevent that, maybe even shutting off the breaker if it happened.
Now, why don’t I install a three prong outlet? Chiefly because that outlet is served by two wire romex and the ground prong wouldn’t function.
Is there a solution? Certainly. A GFCI outlet would shut off the current if the current through the two slots wasn’t equal. Why don’t I install one of those? Because I’m not that comfortable with working live and don’t want to shut my house off when people are constantly on computers. Also, that outlet is daisy chained with another device. and I will have to test to see which set of wires the power is coming into the box on. If the GFCI won’t even fit in the box, a possibility since the old box is small, I’ll have to call an electrician because I don’t know how to fix drywall.