Line weight can be a great way to add volume to your sketch. I have often seen two drawings of the same subject, say, a cup, a sneaker, or a flower pot. The one that uses thicker line weight and thinner line weight looks a lot more clear and more realistic than the one that looks like everything has one thick line all around. The darker lines tell the eye where to look. The lighter lines keep the form open and soft. When every edge has the same heavy outline, the sketch can look flat even if the shape is mostly correct.
So, let us place an object on a table and draw it with only one line weight. One thin dark line on the outer edges, the inner edges, the shadow edge, and also the lightest detail edge. Then you step back and take a look at it. Is it legible? I hope that the answer is yes. Does it look too stiff and like a paper cutout? Most likely it has. Why? Because the viewer cannot clearly read where the closest edges are, where it turns further away, or where the shadow begins and where it ends.
Now, let us draw the same object on the table but with a different intent. Start with very pale construction lines for height, width, and main angle. Keep these lines light enough so that they can remain in your drawing. After getting a good overall shape for the form itself, let us now focus only on the most important darker lines. For example, you can make the closest dark line darker. Or maybe you can also make the shadowed edge of the side of the table where it is darker. You can also emphasize any overlapping edges. For example, the edge between the handle and body or the top edge where one cylinder is overlapping a cube. These few details will immediately make your drawing more readable and clear.
Line weight should have meaning. A heavier line can indicate that the edge is closer. It can emphasize the edge of a handle and its connection with the body of the cup. It can emphasize the dark side of the shadow. And of course, it can also emphasize the dark area of the table where the shadow sits on. It can also be seen as the base of the jar or cup. And finally, it can emphasize the edge of a cylinder or cube that turned further away. Or maybe the dark area where the handle is overlapping the body of the cup. If all of them are just emphasized with dark strokes, you lose some of the meaning behind it. If you choose to only emphasize the closer parts, it would then make your drawing a lot more readable.
It may be helpful to have a quick exercise sheet where you just place different small boxes, cylinders, and simple household objects on a desk. Then, you draw them, but this time you try to mark all the different areas of line weight. So, maybe you put the thinnest line for the lightest construction or planning lines. You put the middle pressure for ordinary visible edges. Then the thickest line for where it touches the table or is turning further away from us. The shadow line. And maybe even some overlapping lines in the drawing. It may not look very nice, but in time, it trains you to just think about it instead of drawing without any thought.
This is also a problem when using pen or ink. The fineliners and markers cannot adjust line weight because you cannot change it with your pressure. But maybe you can get around this by going over the selected darker edges twice or adding a stroke slightly thicker. Then, some of the thinner lines might look a lot better with open edges. The same is true for pen drawing. You don’t need to make each line the same width. Maybe some of the edges are more quiet and other edges define the form a lot more.
It does not matter too much. As long as there is a reason behind it. If you squint to a drawing, you will see how the thicker edges can sit the drawing more and how the darker line is more important and more visible. It also shows you what to look at first. If this line weight can be useful, then your outline is not an edge. It can describe depth and overlap as well as shadow and pressure. Even a sketch can then have more space to breathe.