Because they have such irregular, relatively non-circular orbits compared to the planets, asteroids and comets are much more likely to cross the path of any one of the major planets in our solar system than any other body circling the Sun.
That's right, at any given time, one of these wandering rocks or steaming piles of gas and rock could cross the path of the Earth and collide with it. The effects are not pleasant as I'm sure you can imagine.
This has happened many times in Earth's past, way more than is happening now, but that doesn't mean we shouldn't be on the lookout for future collisions.
The scientific interest in comets and asteroids lie largely with the idea that they are all that's leftover from when the solar system was initially formed. They are the debris field that didn't make it into a planet or the Sun. Over time, their numbers will decrease in only one of two ways: either they are flung out of the Solar System by some close encounter with a planet or the Sun.
Or, they hit a planet or the Sun and get smashed to bits.
Because of this, they are interesting to you and me for a very practical reason: if any one of those things hits us, we are in for a very bad day. All of the mass-extinction events that have occurred in Earth's history can be traced either to a climatic change or a huge something smashing into the Earth and wiping out a majority of species roaming the surface.
Now, I'm the last person you'll see running through the streets screaming, "The sky is falling!", but I also appreciate statistics and I know there is a non-zero chance that something - probably an asteroid or comet - hitting us at some point. I think it's prudent we find as many of them as we can as early as we can.
I also believe in risk-management. In the same way that the numbers of my dying in a car accident are 1/2,500 in the state of Maryland, and I also know that one-third of those accidents are due to speeding drivers. By simply not speeding, I increase my chances of not being killed in an accident by one-third.
That means it's worth my time NOT to speed, it has greatly increased my chances of living while driving to work.
For the same reason, it makes sense to keep an eye out for giant, wandering chunks of rock and ice heading our way.
NASA's Near Earth Object Program is a good lesson in risk management. If we have enough warning that an asteroid is headed towards us, then we have a better chance of survival and mitigating the threat. It's the same idea as hurricane forecasts, knowing where it will hit can save thousands of people.
Every year, it discovers more and more objects to track, and it does so with a network of ground-based observatories and wide-field surveys such as the Sloan Digital Sky Survey.
While NASA tries to give the most accurate information it can, the press releases can be interpreted improperly by the press or misunderstood by the public.
The Apophis asteroid is a case in point. It is currently the only known asteroid of it's size to come so close to Earth since we started taking observations. Although it won't collide with the Earth April 2026 as initially predicted, depending on exactly how it passes the Earth (and how close) during that flyby will have a profound effect on its return in 2036. If it passes through something known as a 'keyhole' then it will hit the Earth. If not, then it'll just be a pretty sight as if flies over our heads.
Quite a difference in outcomes. But there were many out there mis-reporting and stirring up anxiety over the subject. A calm look at numbers and observations is what's required and that's what you'll get here.
Searching, finding and tracking asteroids and comets is important, probably more than any other natural threat, they are the most likely to cause a mass-extinction event here on our humble planet. Something I think we'd all like to avoid if we can, but we want to be honest in our approach and not jump to conclusions and act only according to observations.
Below are some links to articles and videos I've written on this subject. Please feel free to contact me with any information or questions you'd like to see explored more.