Add-on vs. Initial Buy Alerts

Good evening!  Can you help me in understanding the difference between initial buys and add-on buys? And why it is not recommended to make an add-on someone's initial buy?  I think I know why, but I wanted to align my thoughts and assumptions with what you have to say about this "temptation" to buy (if it's initial). I acknowledge that I trend toward being impatient and I must learn to be MORE patient.  R/S, Eric