It tries to explain Quran so that the understanding of the reader about Quran and its message increases. Reading only the translation can give the literal meanings of the words of Quran, however, Tafsir comes with context in which a particular Ayah was revealed, hence giving a context to each and every word, which consequently means a better understanding of Quran and its verses. If you want to read Quran with its translation, you can enroll for the online Quran Translation course now and start knowing the verses with their meanings.
Tafsir is also important when it comes to driving out the laws of Islam from Quran. Verses of Quran come with instructions, and then there are the hadiths of Prophet (PBUH) that also gives instructions. Thus, Tafsir combines both and gives a complete and comprehendible set of instructions to the reader.
When there is no science involved in the interpretation of Quran, then ambiguity and contradiction is surely to rise. When there is Tafsir of Quran available, that contains the scientific method of analysis of verses, the chances of ambiguity and conflict decrease. Hence, instead of ambiguous interpretations there are clear explanations.