Saudah was initially married to Sakran bin Amr, but when she came back with him from Abyssinia on the occasion of the death of Khadija, Sakran died a few days later.
Since Saudah had nobody to take care of her, the prophet married her with careful consideration.
Marriage to Saudah was an act of kindness and a convenience.
When the prophet divided his time equally between Saudah and Aisha, Saudah used to give up her time so the prophet could spend more time with Aisha