FAMILY INTERMARRIAGE!!!!!!!!!!!!!!!!!!

Hello, I have come to the attention that this is something i need to ask for questioning. As my family knows that in our entire pedigree is really intermarriage my parents, grandparents, grt grandparents, etc... I was wondering why would people marry inside family? I was always confused. I do find it very fascinating though when doing genealogy especially when doing genealogy. because its really fun playing around with the intermarriage. I think of it as a giant puzzle and trying to put it together. Although I did find it confusing at first but then learned alot. I was just wondering why was this a custom or practice? My family still does this. I did also hear someone said if you practice Incestry then the child will become deformed or something like that. But we all have came out perfectly fine. My grandfather said something in the early colonial Mexico they Espanoles would marry inside the family to keep something he called quote "La puresa de sangre" although i do not know what that really means at this moment. He said that they didnt like to mix with Indians or Mulatos. Which is prob a little racist. I hope I can get a response. Thanks for all your help in the past. Thanks to this group I have already made connections with over half tof the group so far. Through the Lopez de Elizalde, Ruiz de Esparza, Trevino, de la Garza, Mendoza, and all the other lines etc.
-Daniel Mendez del Camino