Commentaries are just further explanation of meaning and details of the Suttas. Each explanation is subdivided into more details.
E.g. If I ask you to come here. This can be the instruction in the Sutta. The commentaries will go on saying if you are facing another direction to 1st turn towards me, then start taking steps towards me, then when you are at a reasonable distance stop. The reason I might have called you and in which situation I would have called out.
If you can understand and follow the Suttas you need not rely in the commentaries. May find it difficult though. Sometimes certain aspects are explained elsewhere in other Suttas. So doing so might require a scholarly understanding of all related material.
You should take commentaries with a pinch of salt as there can be errors, omissions and miss interpretations like with any other publication / book. Otherwise they can be useful to get and idea of the social customs, background or event leading to the preaching. Also they consolidate from other areas in the Tripitaka too in some cases which can be useful.
When practicing something the final litmus test is to compare with the insight you get at the experiential level. This would help find any inconsistencies. Also some of the later commentaries have correction mentioned about the previous ones. This can be also put to the same test. The errors are very minor though they can be a stumbling block, if you hold on to the belief too strongly. Hence they should be used for additional guidance and information but again being diligent about what you pickup from the source.