What would be different if humans had broader visual spectrum?

Somebody on Worldbuilding (about developing fictional worlds) asked how having a broader range of vision, into the infrared and ultraviolet, would affect what people see. Having a tiny bit of relevant personal experience, I answered:

I have aphakia, so I can address the near-UV end from personal experience. For the near-IR, I can only reason from other sources.

In daylight, subtle effects like the patterns on flowers mentioned in another answer are completely dwarfed by the giant UV source in the sky. Somebody seeing into the near-UV sees sunlight as brighter than others do, to the point of being painful at times. On bright sunny days, or bright winter days with snow cover, I have to wear protective lenses and/or squint to block out a lot of the light. Near-UV vision is a negative in broad daylight.

Near-UV shines, so to speak, in moonlight. Moonlight is just reflected sunlight, but the full moon is only about 1/400,000 the brightness of the sun, so someone who sees into the near-UV can see by moonlight without it being painful. My personal experience is that moonlight makes everything brighter (compared to what I see through glasses) and whiter/bluer. I've never looked for, or noticed, flower patterns or vole urine trails, so I don't know if that's farther into the UV or bees' and predators' eyes are different in this regard.

I would expect a human who can see into the near-UV without the loss of visual acuity that comes from aphakia to see well by moonlight, preferring moonlight to low levels of artificial light. (For example, when walking outside he'd probably prefer not to carry a small lantern.) Light-colored surfaces will be especially visible; he'll probably trip over dark obstacles the same as anybody else.

At the other end of the spectrum, near-IR vision will see thermal signals, the hotter the brighter. This is the principle used by night-vision devices that help military people see opponents. It's not so much an optical "seeing" like near-UV but, rather, heat made visible in near-IR. You'll see a person but not the details of what he's wearing, for instance. (Near-IR vision won't help you see stuff that's the same temperature as its surroundings.) I speculate that a very hot object could "blind" you in the way that bright sun "blinds" you in near-UV, and that near-IR vision will brighten things overall in daylight, particularly on a warm sunny day.

Things like remote controls and IR-illumination LEDs built into security cameras operate in the IR spectrum, so somebody with near-IR vision should be able to see them at least in a dark room. I don't know whether they would be too faint to see in a bright room for someone seeing both normal and IR spectra. I was unable to find anything that addresses how near-IR vision would affect what you see in daylight.

You're extending the range in both directions, so we need to consider how these two modifications would interact with each other. In bright daylight, as best I can tell, neither will confer an advantage and at least near-UV confers some disadvantage. (I think near-IR would confer a disadvantage too.) At night, you should be able to benefit from both; the reflection and fluorescence of near-UV is a completely different process from the thermal properties of near-IR. I was unable to find reliable information about how night-vision devices perform under full moonlight, which would give us some hint about whether they interfere with each other.

People who see in the near-UV and near-IR spectra in addition to normal visible light will probably be super-sensitive to sunlight. I would expect them to either spend less time outdoors or wear eye protection akin to sunglasses or chemically-variable lenses. In darker settings and under moonlight, however, they'll do better than normal humans and wouldn't want any filters in the way. If your humans solve their sensitivity problem with special glasses, therefore, they'll need a way to turn it off. (Sure you can just take the glasses off, but that leaves you with something you need to carry around and protect. Photochromatic lenses appeal to people who don't want to carry around an extra pair of sunglasses; your people will have an analogous problem.)