It is effective because it uses a rhetorical device to appeal to logic
The speaker is quite nervous.
Think of when people are married and sometimes one of the spouses gets "cold feet". It is not because they are sick or cold or even old. They are just nervous.
From when I researched it, I found that he (he being Largo) observed the gestures between them because he believes that Desdemona had an affair, and needs to convince Cassio of it. I hope this helps! ~Mia
