The word את, according to some midrashic approaches, can serve as a marker of inclusiveness. That is, the word את signifies that more is included in the statement than is explicit. See, for example, Ramban on Genesis 1:1.
According to the Mechilta of Rabbi Shimon bar Yoḥai (on our verse), את איש means that liability is only incurred when the ox intended to kill the man, and את אשה means the same for a woman. Furthermore, the word את signifies that liability is incurred for goring a minor, a tumtum, or an androgynous (none of which are necessarily implied by איש or אשה).
The Or HaḤayyim (on our verse) says that את includes liability for an ox that intended to kill one person, and killed another.
According to the Meshech Ḥochma (Mishpatim [24 on Sefaria]), it doesn't say שור איש (without the את) in order to include the case of an ownerless (הפקר) or consecrated (מוקדש) ox. (Perhaps this means: if it read שור איש, we may be tempted to understand this as "a man's ox", ignoring the cantillation.)