Free indirect speech is something I use extensively in my novel, I think part and parcel to the 3rd intimate. I also use direct thought. And achieving the full effect is still something I'm refining/cleaning up stragglers. Which starts to go in another direction in this conversation, LOL.
A third person narrative extensively using the device of free indirect speech helps me as a reader to forget about the outside narrator. I'm able to feel like I'm in the skin of the character, even though the pronoun used in the text is "she" or "he" instead of "I." The narrator is not intruding.
Even with first person narratives, the narrator can intrude. I just read an excerpt on this forum where the first person narrator intruded while telling her own story.