Due to various reasons Bulgarian or Russian phpBB posters type their replies in forums in their native language but using Latin characters. Most often this is because they use a computer that doesn't have Cyrillic support enabled. lat2cyr comes into the picture to automatically transliterate the Latin characters into Cyrillic ones to ease the readers.

Transliteration is a complicated task. There are a lot of things to consider, like URLs, emails, English words or sentences mixed in the posts - those should not be kept as they are. Also there are a whole lot (tens of thousands!) of different styles of romanisation (using the Latin chars to type in a Cyrillic language) and those should be accounted for.