You can find the explanation in page for Pinyin on wikipedia:
Note on y and w y and w are equivalent to the semivowel medials
i, u, and ü (see below). They are spelled differently when there
is no initial consonant in order to mark a new syllable: fanguan
is fan-guan, while fangwan is fang-wan (and equivalent to fang-
uan). With this convention, an apostrophe only needs to be used
to mark an initial a, e, or o: Xi'an (two syllables: [ɕi.an])
vs. xian (one syllable: [ɕi̯ɛn]). In addition, y and w are added
to fully vocalic i, u, and ü when these occur without an initial
consonant, so that they are written yi, wu, and yu. Some
Mandarin speakers do pronounce a [j] or [w] sound at the
beginning of such words—that is, yi [i] or [ji], wu [u] or [wu],
yu [y] or [ɥy],—so this is an intuitive convention. See below
for a few finals which are abbreviated after a consonant plus
w/u or y/i medial: wen → C+un, wei → C+ui, weng → C+ong, and you
→ C+iu. Well, this note is so detailed, I really feel it
difficult to add somethi