In the United States there are three general avenues by which people can immigrate. Likely the easiest way is to apply for the Green Card Lottery simply because of its few requirements for maximum benefit (permanent residence). The other two categories that allow you to get permanent resident status often take a long time to put into effect. The employment based green card requires that you find a job and get hired before you even immigrate and the family based green card is only useful if you have family in the United States. However, even if you have family in the United States it might take ten years to get a green card through this process, so it makes sense to apply for the Green Card Lottery while you wait.
The justification behind immigration, at least from a theoretical standpoint is a point of contention within the two major political parties in the United States. It’s a difficult thing to define anyway, but the thinking behind it is this: whenever a country brings something in it has to somehow benefit the country, the best way to identify a good is to qualify some reasoning, a continued benefit is better than a static one. So, when the government allows people into the United States they prefer the immigrants to be educated and able to do a skilled job. However, because there is a distinct focus on maintaining family values in America, they also allow family members to immigrate to reunify families. They also allow people to enter the United States to diversify American immigrant numbers.
Currently, more conservative thinkers believe that it is preferable to only grant visas to people who can provide a direct economic benefit to the United States by working and providing skills to the job market not already taken up by Americans. But the problem with this line of thinking is that it discounts the necessary trust one should feel in other human beings. All you really have to do is give someone a chance to show their ability and give it their all in the greatest country in the world and the decision will pay dividends.