In general, the most important function of the Zionist "pro-Palestinians" is to enforce two boundaries in the discourse:
1) the legitimacy of Israel as a Jewish state;
2) the illegitimacy of violence against Israelis.
The first of these boundaries seeks to legitimise or ignore the ethnic cleansing of Palestine by the Zionist movement in order to establish Israel as a state for Jews with a Jewish majority. The second seeks to legitimise the massive use of force against Palestinians. So that's what the Zionist left is for.
This needs to be exposed. Much has been written about anti-semitism rearing its head on the left and even in Jewish leftist circles but very little is said about the more than passing flirtation of some leftist groups with Zionism.