The assistant system allows you to have a little help while drawing straight lines or circles.
They can function as a preview shape, or you can snap onto them with the freehand brush tool. In the tool options of free hand brush, you can toggle 'snap to assistant' to turn on snapping.
The following assistants are available in Krita:
A assistant for drawing ellipses.
This assistant consists of three points: the first two are the axis of the ellipse, and the last one is to determine it's width.
This ruler takes four points and creates a perspective grid with that.
This grid can be used to create with the 'perspective' sensor, which can influence brushes.
There's three of these:
This ruler uses four points to create a cubic bezier curve that you can draw along.
This assistant allows you to create a vanishing point, to which a preview is drawn, and all snapped lines are drawn to.
It is one point, with four helper points to align it to previously created perspective lines.
They are made and manipulated with the Ruler Assistant Tool.