I have an umlaut as part of my family name. It sucks that in most official documents they never include it.
In Spanish, the combination of
'g' and
'e' or
'i' is special. For other vowels, there are two sounds,
'j' and
'g'.
For example,
'ja' would sound something like
'ha' (harsh sound) while
'ga' sounds like
'gah' (soft sound). Same with
'o' and
'u'.
But for
'e' and
'i' the sound is always harsh
'ge' == 'je' which sound like
'heh' and
'gi' == 'ji' which sound like
'hee'.
To get the softer sound, you need to put an
'u' in between:
'gue' or
'gui', resulting in
'geh' and
'gee' (more or less).
The umlaut is used when the
'u' should be pronounced normally in those instances, so
'güe' sounds something like
'gooeh' and
'güi' sounds something like
'gooee'.
Edit:
Wikiquote!Several languages use diaeresis over the letter U to show that the letter is pronounced in its regular way, without dropping out, building diphthongs with neighbours etc.
In Spanish it is used to distinguish between "gue"/"güe" and "gui"/"güi": antigüedad (antiquity), pingüino (penguin). This is also the case for the Nicaraguan demonym, Nicaragüense. Similarly in Catalan language,
"gue~güe" are [ɡe]~[ɡwe],
"gui~güi" are [ɡi]~[ɡwi],
"que~qüe" [ke]~[kwe],
"qui~qüi" [ki]~[kwi],
as in aigües, pingüins, qüestió, adeqüi. Also ü is used to mark that vowel pairs that normally would form a diphthong must be pronounced as separate syllables, examples: Raül, diürn.
So yeah it is a little complicated.

Back on topic, I guess the umlauts are used for visual effect; Adding uncommon symbols makes the text stand out from other game titles.