Why did scientists need to invent light years? Why did scientists need to invent light years? What's so important about having a light year? I have been learning that a light year is $9.461 \times 10^{15} \, \mathrm{m}$. My question is, why are light years so important for other people to know. What do scientist use these methods for?
 A: Whenever you see things at a distance, your perceptions are of things that happened in the past. You can see it at a football game, for example, where someone kicking the ball is seen very much before it is heard (sound is slower than light). 
Light only has a finite speed, too; and light turns out to be the fastest thing there is. A light year is the distance that light travels in one earth year. We could also have light minutes, or light seconds, for closer things.
When we tell you how far away another star is in terms of light years, we are also telling you how many years ago the light was emitted. So when we say that the Andromeda galaxy is 2.5 million light years away, we're really saying "all of our images of the Andromeda galaxy show how it was 2.5 million years ago." We're also saying "it would take at least 2.5 million years to get there, no matter how you traveled there," since you can't go faster than the fastest thing there is. 
So the nearest star to our Sun is called Proxima Centauri and is 4.24 light years away. If we sent someone over to Proxima centauri and we wanted to talk to them, we would have to send a message by light, which would be "opened" four years and three months later, and their response would come back to us four years and three years later. When we speak of light years, light minutes, light seconds -- all of these time units give you a sense of how hard it is to keep up a conversation with a person or satellite at the other end of that. We use human units like the "year" because we are human and want to compare things to, say, the human lifespan. 
In the case of Proxima Centauri, we know for example that we're likely not even going to get our rockets to 1% of the speed of light in the next century, so any trip to send a human explorer out that far would have to send a small dynasty/nation of people who could work together for 500+ years. There is no hope for that anywhere in the next thousand years or so.
Mars, on the other hand, tends to be about 12 light-minutes away -- much smaller than a year, so we'd never use light years to measure that distance. We can't interact with our satellites around Mars much faster than waiting half an hour for us to send a message, for the satellite to do whatever we told it to do, and then for it to send a message back.
A: Ultimately, the answer boils down to convenience.
When we want to describe the distance between here and, for instance, the star Sirius (the brightest star in our night sky), it would be a little cumbersome to write $\ell=8.13\times10^{18}\,{\rm cm}$ any time we want to write its distance from us. And really this goes for any astronomical object: they're just too far away to use the centimeter (which is how any respectable astronomer write their distances, though "rules" say we shouldn't).
Instead, we actually have two "large" base distances at our disposal:


*

*lightyear at $9.46\times10^{17}\,{\rm cm}$

*parsec at $3.08\times10^{18}\,{\rm cm}$


The latter of the two is more common in astronomy publications (at least the ones that I've read). Either way, each of these allow us to write that Sirius is 2.65 pc (8.6 lightyears) without resorting to littering publications $\times10^n$ notation everywhere.
A: Light years help give an idea of distances through space and time. When we look at a star 100 light years away, that 100 light years not only gives an idea of the immense distance to the object but will also tell us that what we see is light from 100 years in the past.
