In Nahjul Balaghah,the first Shia Imam, Ali ibn Abitaleb (AS) described various benefits of Islamic unity of which the main ones are outlined briefly.

1.Protection of the religion of Islam- In Letter 62 of Nahjul Balaghah, Imam Ali (AS) wrote: I stayed back until I saw a group that left Islam wants to destroy the religion of the Prophet Mohammad (PBUH). So I feared that if I do not help Islam and its followers, I could witness invasion/infiltration or its destruction which its calamity would be harder on me than to abandon the rule over you. Then in the middle of that chaos and uproar, I rose up until the falsehood was destroyed and the religion established and settled.

2.Preventing tribal and religious discords- In Sermon 176, Imam Ali (AS) said: Avoid division in religion, as unity in the way of the truth even though you may dislike it, is better than division in the way of the falsehood even though it is your favourite; because Allah (SWT) has not granted anything to ancestors or the future generations through division.
Other benefits include:

3.Establishment of the Islamic government

4.Establishment of Islamic justice

5.Enjoyment of Divine Blessings

6.Preventing Satan’s domination

7.Victory for Muslims

Source: en.shafaqna.com