I really don't understand why Spanish reserves the letter 'w' for words acquired from other languages.
The letter ‘W’ wasn’t part of the original latin alphabet. It was the last letter to became part of the Spanish alphabet, the Academy officially recognized it in the 1969 orthography. It was created in the german languages trough the duplication of the latin ‘v’. It was taken as a loanword already in the Middle Ages, to write Germanic words, most of those words now are written with proper Spanish letters like ‘g’ (gualda, güelfo) or ‘v’ (vagón, váter). It is now used in loanwords like web, sándwich, waterpolo, Wagner, Weimar, etc.
The letter doesn't properly belong to the actual spelling of the Spanish language; indeed, it is only normally used to spell out loanwords, especially from gothic languages, German, and English, and Latin language transcriptions for words from oriental languages.
The phoneme /w/ from latin became /v/ in the romance languages; for this reason V ceased to be appropriated to represent the sound /w/ of the Germanic languages.
I guess you could compare it to the letter "ö" from German; the English spelling for this letter becomes "oe", such as in "Schoenberg (Schönberg)", and it's used almost exclusively when spelling out German words.