Sure you can draw line with the line tool. But I have never felt confortable with that, I think that’s because the resulting line looks too perfect, as you can see on the screenshots below (it’s the top right line).
What I want is to have the virtual version of physical tools (rulers, compas, we can even imagine more fancy stuff). That’s what a drawing assistant is. It controls the drawing on the image, depending on the mouse/stylus movement. So for the ruler drawing assistant, it will force to draw a line. There is a control over the magnetism of the tools, this control how much freedom the artist has, how much the pen can move aside the ruler line.
The second line (starting from the top right), was done using a mouse. The third and fourth with a tablet stylus. The fourth line is here to show an other interest for the drawing assistant, you can control the pressure all over the line.
In fact, I am more interested in a compas tool, but a ruler was more simpler to test the whole framework.