I'll let Lawrence Jarach answer this question, from this essay "Don't let the Left(overs) ruin your appetite" found here: <http://theanarchistlibrary.org/HTML/Lawrence_Jarach__Anarchists__Don_t_let_the_Left_overs__Ruin_your_Appetite.html#toc6>
"An uneasy relationship has existed between anarchists and leftists from the time Proudhon positively proclaimed him self an anarchist 150 years ago. From the 1860s through the 1930s most anarchists considered themselves to be an integral part of the international labor movement, even if there were moments of extreme conflict within it; leftist anarchists saw themselves as the radical conscience of the Left — the left of the Left, as it were. But since the death of 19th century anarchism on the barricades of Barcelona in May 1937, anarchists haven't had a movement to call their own. As a result, many anarchists trail after leftist projects, seemingly oblivious to the sometimes fatal historical rivalry that has existed between the two tendencies. They get seduced either by the seemingly antiauthoritarian characteristics of such groups (like decentralization), or by the use of some anarchic vocabulary (direct action for example).
The Left has consistently been identified with the international labor movement from the time of the First International; with the shift of focus from western Europe toward Russia beginning in 1917 and continuing into the 1960s, leftists have identified themselves in relation to events that occurred in the workers' paradise. Whether a leninist, trotskyist, stalinist, or non-leninist communist, each variety of leftist has a particular view of when things went wrong (or not) with the Russian revolutionary experiment.
A return to authentically anarchist principles, coupled with some understanding of the troubled history of the relationship between leftists and anarchists, can go a long way toward reinvigorating antiauthoritarian theory and practice. At the same time, moving beyond the melioristic beliefs (especially about western European technology, culture, and science) of 19th century anarchism, which have made the programs of anarchists and leftists seem similar, is crucial. The relevance of anarchist self-activity can only increase when the vestiges of authoritarian leftist assumptions and distortions are discarded from the words and behavior of antiauthoritarian activists, critics, and theorists."
(By the way, I dare you to tell me that this is not a "post-anarchist" essay)