In Bal Kand, Mahavishnu after pleadings of Devtas decided to incarnate in House of Raghu to destroy Ravana. Mata Sita too is called consort of Vishnu (Mahalaxmi) many times in the epic.
The reason of exile of Shri Ram was Kaikeyi’s ambition to forever remain relevant in the Kingdom. She had no problem with his coronation, she had problem with Kaushalya gaining more power and prestige than her. If only she had discussed her insecurities with Dashrath rather than panicking and wanting her boons, many things wouldn't take place. Shri Ram understood that Kaikeyi won't stop till she has achieved her goals so to please her , he left for the exile although Laxmana was against it, he said he thinks the destiny wants him to go in this exile.
The ambition of Kaikeyi was the reason for exile of Shri Ram historically and devotionally , MahaVishnu incarnated to destroy demon Ravana who made the world cry.