Papeeria keeps text files in UTF-8 and that's okay for most of the users. They either write in plain English or are happy with UTF-8 support in the modern TeX. However, there are legacy packages, document classes and bibliography styles which work best when user input is in a "legacy" 8-bit charset, such as Windows 1251 or KOI8-R, and some people need those packages for compiling. Usually documents which may need legacy charsets have something like this in the preamble:
We've added just two charsets which are the most popular in the Russian TeX community. If you need other charsets, please let us know.