One of the problems is that some characters have multiple encodes. It's a problem if you enter one form in one computer and another form in other.

From: https://en.wikipedia.org/wiki/Unicode#Ready-made_versus_comp...

For example, é can be represented in Unicode as U+0065 (LATIN SMALL LETTER E) followed by U+0301 (COMBINING ACUTE ACCENT), but it can also be represented as the precomposed character U+00E9 (LATIN SMALL LETTER E WITH ACUTE). Thus, in many cases, users have multiple ways of encoding the same character. To deal with this, Unicode provides the mechanism of canonical equivalence.