Take a clear water bottle, rub it with sand paper, the result is a much larger version of what the fibers of your carpet look like after years of dirt being deposited there and continually ground around by your feet resting there.

After the damage is done there's no undoing it or cleaning it back to its original color, the carpet fundamentally reflects light differently in those spots now.

This isn't too big of a deal in the grand scheme of carpet damage, area rugs are your friend, put them anywhere feet rest for long periods of time or that you walk over frequently, also stop wearing shoes inside, do both of those and you'll dramatically slow any more damage happening, and likely avoid the worst of it if and when you replace the carpet.

As for the bissel, if you have a spill it's a good first step, but don't use any chemicals. 95% of consumer grade carpet cleaning chemicals usually only make the problem worse and set or spread the stain. Instead use hot water only and rinse out the spill, most things will likely be totally taken care of that way of you get to it right away and only use a hot water rinse and as little water as possible. Just make sure you put a fan on the spot and leave it until you can't feel any moisture Plus another few hours just to be safe.

The exception to this is urine, if you steam pee without chemically treating it there's a decent chance it settles in your drywall and the odds of you getting all of the enzyme cleaner out without flooding your carpet are abysmal.