They are two different vowels. There are 8 vowels in Turkish. All letters correspond to a different phonetic sound in the language. So you can read it as it is written. Iı sounds like e in ‘the’, and ‘i’ sounds like ‘ee’ in deep. They are different sounds, different vovels hence conveniently different letters. Difference changes all the meaning. For instance “sınır” means boundary and “sinir” means nerve. I think it is simple and brilliant design for Turkish. I respect the design choices of all the other languages but this is what works best for our own language. You can simply accept that it is a different (modified latin) alphabet for a different language.