It is not necessary to include these relationship words but they can be very helpful. If one can read two ideas connected by a relationship word and if the resulting compounding of the sentences makes sense, then you can feel quite sure that the ideas relate to each other. If the reading of the two sentences connected by a relationship word does not make sense then you know that the ideas do not imply each other. You are required to have two sub-points for every main point when you are outlining topical and sequential structure. Topical structuring is classification that divides ideas into at least two sub-points.
Sequential structure places ideas in a sequence and it takes at least two ideas to have a sequence.
However, if you are using logical structure and one idea is used as a reason for the other you may have one sub-point or many. You will want to separate the reason from the point it supports whether you have one or many reasons supporting that conclusion. The same is true with analogical structure that employs comparison or contrast. You may have one point of comparison or you may have many. You will want to separate the point of comparison from what is being compared.