“The ācāryas have explained in various plausible ways why Lord Kṛṣṇa took His wife Satyabhāmā with Him. Śrīla Śrīdhara Svāmī begins by saying that the Lord wanted to give His adventurous wife a novel experience and thus took her to the scene of this extraordinary battle. Also, Lord Kṛṣṇa had once granted the blessing to Bhūmi, the earth-goddess, that He would not kill her demoniac son without her permission. Since Bhūmi is an expansion of Satyabhāmā, the latter could authorize Kṛṣṇa to do the needful with the unusually nasty Bhaumāsura.
Finally, Satyabhāmā had been miffed when Nārada Muni brought a celestial pārijāta flower to Queen Rukmiṇī. To pacify Satyabhāmā, Lord Kṛṣṇa had promised her, “I’ll give you a whole tree of these flowers,” and thus the Lord scheduled this procurement of a heavenly tree within His itinerary.
Even nowadays devoted husbands take their wives shopping, and thus Lord Kṛṣṇa took Satyabhāmā to the heavenly planets to get a heavenly tree, as well as to retrieve the goods Bhaumāsura had stolen and return them to their rightful owners.
Śrīla Viśvanātha Cakravartī notes that in the heat of battle Queen Satyabhāmā would naturally become anxious for Lord Kṛṣṇa’s safety and pray for the battle to end. Thus she would readily give permission to Kṛṣṇa to kill the son of her expansion, Bhūmi.”
Source:A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Tenth Canto, Chapter 59 – Text 2-3