A powerful insult. In fact, it is twice as insulting as using only the word maggot or faggot alone. Also, it rhymes; rolling off the tongue nicely and adding an element of cool suave to an oththerwise vulgar slur. It can be reversible to "faggot maggot", too - just to switch things up a bit. Best served as loudly and aggressively as possible. Extra points for utilising a piercing red face or impersonating the most hostile, homophobic army sergeant you can muster.
Worthless person: *begins to speak*
You: “SHUT UP MAGGOT FAGGOT!”