the Origin of this kind of writing is Counter STrike , much names where blocked their etc , but u was able to take the name of someone other if u edit the letters to numbers for example
Crusher
so someone wanted to be named Crusher but since this name is taken he need to change the letters to anything different to can wear the name, so he change it to Crush3r
but i think the Origin is the graffity scene , sprayer had nicknames and replaced numbers or symbols with letters , such as the graffitystyle is, u also can see this on gangs for example Bl00dz, Cr$bZ,M513
i can show u all synonymes for letter
A = 4 @ ^ /-\ α
E = 3 €
G = 6,9
I = 1 | ? !
L = 2 /_
O = 0 Ω Ф
R = я
S = 5 § $
it ever depends on the looking of the letter what number/symbol u can use , there are much more possibilities , i just wanted to explain u how it works bro ,i hope i could help u =)