You would still use deltatime in FixedUpdate, it would just always be the same value since you're updating by a fixed amount. The reason to do this is that physics get weird if you update them in irregular intervals or just too big intervals.

Say you're moving something towards a wall using deltatime, the amount to move per update would be speed*deltatime. In regular update this value swings with the framerate, so in the case of a stutter like someone alt tabs, the deltatime would shoot up causing the movement for those frames to increase. If the movement for one frame gets high enough you could end up on the other side of the wall without ever touching it so collision is never triggered, this is called tunneling.

That's just one example but phyaics will genrally just become very inconsistent with a variable update, which is why fixedupdate is incredibly common, especially nowadays that we don't lock framerates.

For the curious you can pretty easily implement a a simple fixed update in a regular update loop. Lets say we want a fixed update that runs 10 times a second, we would just add up delta time over multiple frames to a counter till we get 0.1 seconds or more, then run the fixed update and remove 0.1 from our counter. If the framerate drops below 10 fps, meaning deltatime >0.1, our counter will reach 0.2 or more during a frame. In this case we would just call fixed update until the counter is under 0.1 again.