In the history of the letter 'v', Wikipedia mentions the origin of 'u' but unfortunately doesn't describe why it was created in the first place:
During the Late Middle Ages, two minuscule glyphs developed which were both used for sounds including /u/ and modern /v/. The pointed form "v" was written at the beginning of a word, while a rounded form "u" was used in the middle or end, regardless of sound. So whereas "valour" and "excuse" appeared as in modern printing, "have" and "upon" were printed as "haue" and "vpon".
The article on 'u' states the same thing:
During the late Middle Ages, two forms of 'v' developed, which were both used for its ancestor 'u' and modern 'v'.
It's a shame it doesn't go into depth about why the alternative glyph was developed.
What caused 'u' to start being used? Why couldn't ancient writers continue using 'v' for everything? Is there anything else interesting about the history of 'u'?