You need a Y-axis, when you want to add a XY scatter chart series to a region, see Basic concepts for more explanations. After adding a Y-axis object to a region you have accsess to it for setting all needed properties which belong to Y-axis objects. Each region can have one or two Y axes, therefore you must pass an Index when you want to get access to a previously added Y-axis.
Impportant: X and Y axes must always have a counterpart on the other side, they can only exist as a pair of axes. It's not possible for example to have one X axis and two Y axes. For each X axis must exsit a Y axis, for each Y axis a X axis.