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No, that's bad motion blur - TAA would leave a spacetime distortion behind the sword

Tbh the reason to play Elder Scrolls games were it's flaws.

There's two kinds of motion blur, really - camera based, and model based. Camera-based requires calculating one motion vector for the whole screen, which is basically free. Model-based requires projecting the motion of each vertex of the model in the projected view; one matrix multiply per vector is not 'expensive' on a modern graphics card. Depth of field requires comparing the depth buffer, which you'll have already created as part of rendering, and then taking several 'taps' around each point on the screen to calculate the blur for the 'focus distance' compared to the actual distance. The final image post-processing will generally process the whole screen anyway, so you're just throwing a couple of extra steps in for the two effects.

Now, what does it save you? If your engine is using TAA (temporal anti-aliasing) then that's performed by 'twitching' the camera a tiny amount (less than a pixel) every frame. If nothing's moving, then you can merge the last several frames to get a really high-quality anti-alias; all the detail that wouldn't be caught with a 'completely static' camera will be captured, and the result looks great. But things do move; if you recalculate 'where things were' then you can get a reasonable idea of what colour ought to be at each pixel. Since we need to calculate all the movement vectors to do that, then using the same info gives us the motion blur data 'for free' - we can add a little blur in post-processing to hide the TAA mistakes in post processing, and when implemented well(*) then it looks pretty effective. It's certainly much, much cheaper to calculate that 'proper' antialiasing like MSAA.

(*) It is also quite easy to not implement TAA well, and earn the ire of gamers for turning everything into a blurry mess. Doom (2016) does a fantastic job of it - it's in the engine at a low level - and I've never seen anyone complain about that game being blurry or smeared.

It takes time to load high-quality textures and models from disk, and it uses up the RAM budget for each frame. Using lower-quality textures and models for distant objects greatly helps rendering speed and prevents stutter, and a bit of depth-of-field hides the low-quality rendering with a bit of a smear.

Now, if your graphics card greatly exceeds the design requirement (which was probably some kind of console) then you can switch these effects off and the game will look even better, which might make you question why they're there in the first place. To help consoles look better with some 'cinematic' effects, is why.

I'm actually making a computer game that takes place in space, within solar systems and is in 3D with a (somewhat) realistic style - a mistaken choice of art style given the time I'm having to spend in 3D modelling even though it's mostly empty space :/

It would be near unusable with realistic lighting, especially further away from the system's sun, because in most places there's just this one decent light source which is the sun and this extremelly weak "lighting" from the background which is mainly black with stars (since it doesn't take place near the center of a galaxy, that background has about the same star density as the night sky on Earth). If I was doing realistic lighting, only near the sun or near planets would it look good, and the latter only on the sunny side of the planet (because of the indirect light from the planet), not the dark side.

So I'm having to fake it using ambient light, otherwise most objects would just be entirelly dark most of the time (it would be that or putting lots of little lights on them, which carries other implications).

Anyways, the point being that most space scenes in Sci-Fi films are also total complete bollocks for similar reasons: almost everything would be pretty much black almost all of the time or at best have very sharp shadows everywhere but near the illuminated side of large stellar objects. (There's actually a scene in Star Wars - Rogue One where they purposefully use realistic illumination for effect and the scene starts with a stary dark background and suddenly a star destroyer starts to emerge from such a total darkness that it was not at all visible, and we see more and more of it come out from the shadow of the Death Star).

In all fairness, in my journey learning game making, lighting turned out to be an unexpectedly interesting subject and actually fun, though it's unusual to have the chance to really use it for effect (well, the rings around the planets in my game do look pretty stunning merely from the interplay of light and shadow as long as I their axis is tilted relative to the direction of the sun).

The director said the lighting was coming from the same place as the music.

hahah that's great