It’s really common cross-linguistically, especially in widely spoken languages, so it’s useful for preserving sounds. /w/ is phonemic in English, Mandarin, Spanish, French, Arabic and Portuguese, and it appears but is not phonemic in Spanish and Bengali. Also, out of the /v~β~ʋ~ⱱ~w/ group, it’s the best to include because it’s the easiest to learn for people who don’t know it. It’s literally just a shorter /u/; one of the most common vowels cosslinguistically.