Oral and nasal vowels often contrast meaning. Look for instance at the following pairs of words:
hí - 'tooth'
híŋ - 'body hair'
ská - 'white'
skáŋ - 'to melt'
k’ú - 'to give smth to sb'
k’uŋ - 'the aforesaid'
tȟáka - 'to parch'
tȟáŋka - 'big'
For this reason, it is important that you train your ear to hear the difference and your tongue to pronounce it.