'Ilm Al-Rijaal is the knowledge about the authenticity of narrators which leads us to evaluate their narrations to decide whether we can believe them or not.
In Quran Allah (SAWA) says: ( قل آلله أذن لكم أم على الله تفترون )Did Allah permit you or you falsely claim on Him?(Sura Younus;59)
Which is very clear that we can not claim any thing in religion of Allah with out authentic evidence. Narrations can be authentic and can be not. That is why we must study the narrators very carefully before taking their narrations as the matters of religion are a big responsibility on us.
We also read in Quran that before believing any narrator we must be sure about his authenticity. (Al-Hujoraat ; 6)
We also have plenty of Hadeeths from the Prophet (SAWA) and Infallible Imams in this important matter. In fact, we can not take any narration in any important matter in life before we trust the authenticity of the narrator, so what about matters of religion?
That is why, 'Ilm al-Rijaal is necessary to evaluate narrations through evaluating the narrators.