zahar0z
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Because the shading looks bad if you don't do it this way. The cuts are made in the places where the pipe is interrupted by windows (or loopholes).
If by "shading" you mean smoothing, then in Max, for example, there is an option to adjust this smoothing, and at an angle greater than 90 degrees, it looks good. I am 99.9999% sure that Blender has a similar function. In short, for a normal bake of the sky on this pipe, you don't need 100 faces, 8-10 are enough, which you can then use as a low-poly model. This is better because when baking 100 faces onto 8-10, artifacts will definitely appear, and it is quite possible that you will have to correct them manually, which is a real pain, and it is easier to just use a low-poly model.
Regarding the windows themselves, this is also a separate issue; there is a high chance that it will bake incorrectly due to such a difference in the number of polygons. I can suggest two options. Either make two pipes (inner and outer) and cut out the windows with an alpha on the texture. The second option is to paint them on the normal map. This is easy: draw a black and white mask with windows on the texture, in Substance Painter, create a layer based on this mask that gives depth, and transfer this depth to the normal map. And then further create a curvature map and beautifully texture it using the normal map. In fact, ideally, you should do both: cut them out with an alpha and create a normal map.
In short, there are many different tricks when working with low-poly models; you need to constantly come up with tricks to make it look good.