The Zn acts as sacrificial or cathodic protection to prevent rusting of iron because:

${\mathrm{E}}_{\mathrm{op}}^0\text{ of }{\mathrm{Z}}_{\mathrm{n}}>{\mathrm{E}}_{\mathrm{op}}^0\text{ of }\mathrm{Fe}$

that means oxidation of Zn→Zn²⁺
 will occur instead of Fe→Fe²⁺/Fe³⁺, thus corrosion is prevented.