[Tex/LaTex] Why to use [T2A, T1] when [T2A] is enough

Question

I am writing some seminar in Cyrillic for university

\documentclass[a4paper,14pt]{extarticle}
\usepackage[T2A,T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{droid}

\DeclareRobustCommand{\cyr}[1]{%
  {\fontencoding{T2A}\selectfont#1}%
}

\renewcommand{\contentsname}{\cyr{Садржај}}
\renewcommand{\refname}{\cyr{Литература}}


\begin{document}

\begin{titlepage}
\vspace*{\stretch{2}}
\begin{center}
\huge
\cyr{Назив семинарског} \\

\vspace{\stretch{4}}
%\normalsize
\cyr{Име и презиме}
\end{center}
\end{titlepage}

\tableofcontents
\clearpage

\section{\cyr{Глава 1}}

\cyr{ЋЧСАЋЧСАЋ АСААС ШЂСАШЂ ЊЉЖ СААЋЖ СЦАБЕК АСАЖЋ сажћ ћафа ћжасжћашђ асдажћ}



\section{\cyr{Глава 2}}

text $math$

English alphabet W Q 

$\int\limits_0^1 \frac{x^2-3x}{\sqrt{x-1}}\,dx$


\section{\cyr{ПОГЛАВЉЕ}}

\end{document}

But when I use

\documentclass{article}
\usepackage[T2A]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{droid}

I don't see any difference. So I don't have T1 fonts included. I mean, here I can also write English alphabet using English keyboard and I write here Cyrillic using directly Cyrillic letter. And here I didn't define \cyr because I have tried different ways.

So mine question is: why should I use first way instead second? I suppose there is different between method 1 and 2, but which one?

Answer 1

The character maps of T2A and T1 are identical in the first 128 positions. So english texts should work fine with T2A. But if you want to insert umlauts and other accented chars from the upper part of the char table of T1-encoding you should consider to switch to T1 for such texts instead of relying on LaTeX to fall back to the correct definitions.