Umlaut occurred in order to make words easier to pronounce.
For example, in Old High German, the word gast 'guest' had the plural form gesti 'guests': the plural ending -i caused the vowel in the stem to be a front vowel e. This vowel alternation remained in the language, so that present-day Standard German displays the forms Gast [gast] – Gäste [gɛstə], although the final front vowel has been reduced to the central schwa vowel.
I-mutation is the most prominent of the processes, to the extent that it is often referred to simply as "umlaut".
[5] Similar processes also occurred in the history of the Celtic languages, especially Old Irish.
[6] Vowel-raising umlaut occurred in the history of many of the Romance languages, in which it is normally termed Metaphony.