The definition of \appto is
% etoolbox.sty, line 891:\newrobustcmd{\appto}[2]{% \ifundef{#1} {\edef#1{\unexpanded{#2}}} {\edef#1{\expandonce#1\unexpanded{#2}}}}The first argument should be a parameterless macro. Why not using \apptocmd? Because it is much less efficient and it may even not succeed if the replacement text has “non standard” tokens (for instance, characters with a different category code than usual).
Apart from this, the only other difference between \appto and \apptocmd is that the latter preserves the status of a \long (parameterless) macro, whereas \appto will always change it to non-\long.