ZModeler is oriented on low-poly modelling and precise control and accuracy of polygonal mesh should always be the aim of 3D artist. Consider you have created a box and chamfered it's edges for smoothness, but faced out the problem that you don't have enough cross-sections for some details:
Missing a multiple cross-sections could be a pain to find out, especially when you want them to be straight and accurate:
In most of cases, precise and accurate slicing could be performed with interactive Insert tool, when you can pick an edge to cross one by one. The straightness is not guaranteed but could be achieved with later scale horizontal or scale vertical on newly-inserted vertices, but this can be used to straighten horizontal or vertical vertices after insertion and will not work when insertion goes on 45-degreese direction. Another disadvantage of manual points insertion is an interruption at sharp edges. Once you reach the sharp edge, insertion is applied and you should start from the neighbor edge to continue slicing.
The Quick Slice tool will slice an entire object (hovered) or a set of selected objects in a single run given a point and direction for the slice plane. It can also slice a selected polygons only, so you can restrict the area of application. The tool has a new vertex insertion tolerance distance that let it reuse existing vertices instead of creating new vertices withing a given distance range. A rotation angle snap is controlled in tool's options too. To turn the angular snap off, hold the Ctrl key. The tool draws the cross-line in a color of "Reference", which is red by default. If you don't like the line color matching the "selection" color, you can adjust custom color for "Reference" in Settings->Colors->Meshes.