HI! practically there's no difference in the sound of the two vowels; still, orthography requires their distinct use.

In greek there are the 'i' vowels and a couple 'i' combinations. So whenever you see 'ι, η, υ, οι, ει and the rare υι, ' know that they sound more or less the same.

One might ask if there's a reason for the existence of so many 'i's. There are indeed a few reasons. It is strongly believed that in ancient greek most of those 'i' did sound individually. In modern greek, although they sound more or less the same they indicate the origin of the words, especially the combined words; other reasons exist too. Greek has the most complex orthography amid indoeuropean languages.