Comparing scripture with other scripture is very important in understanding the Bible. It helps to keep us from making mistakes in our understanding. For instance we may interpret some scripture as to mean some specific thing and then find another portion of scripture that seems to contradict the first. In that case, we probably misunderstand one or both portions.
Comparing scripture with other scripture can also confirm our understanding when we find the same teaching in several places in the Bible. It can also give us a more complete understanding of some subject or topic that we may be studying.