Why does the U.S. government want to keep track of us?
That is rather hard to answer. We can come up with all kinds of glib quips, but basically, most governments seem to keep track of the populations that they presume to govern because that is one of the things they do. For one thing, a government acquires its right to govern through the consent of those it governs. That consent can be coerced or offered. North Korea represents an extreme in coercion, while the US likes to believe, both the government and the governed, that we are a democracy and thus the epitome of public willing consent to be governed. Governments are delineated generally by two key elements. One is a defined territory and the government stops - more or less - at the borders. The other is the population governed. It would seem to be a logical thing for a government to know who it governs. The rules for citizens in the US for example are different than those for visitors. The mechanisms of government have to able to know in some manner what rules apply: citizen or visitor.
For instances when that fails look at the screwy situation in Australia where a law was passed and at some point, they discovered that "oops, we have government positions by people who can't legally hold them because the have dual citizenships." Moreover, many of the "possessors" of said dual citizenships were unaware that they had dual citizenships, and others were unaware that they could not hold office, until they had been at the job because nobody told them not to bother.
Then there are other matters related to law enforcement, intelligence and security. They don't want you breaking the rules you nominally agreed to obey as a citizen, nor to be telling tales to foreign agents that they should not hear, nor commit crimes against good citizens because you have a religious conviction that only your beliefs are worth having and everyone who doesn't share them is surplus protoplasm.
The QUORA is rather broad. Exactly what "keep track" is in play?
On the constitutional level, there is a CENSUS mandate every ten years. This also has an ongoing element connected to births, deaths, marriages (and related name changes) and divorces. There is a running statistical need to know how many people live where.
From that level, you then move to things like selective service, and funding based programs from education to public assistance. Keeping tabs on the elderly ensures social security payments go to the living... and get to them in a timely manner.
After that, there are levels that eventually get to the CCTV level of survalence to prevent crime, or apprehend those who commit crimes in public areas.
Beyond that you have BIG BROTHER... spying and invasion of privacy which go beyond rational needs.
But, on the broadest level...
The government keeps track of you because the CONSTITUTION mandated it as the basis for structure the society and political voice of the people in a representative democracy.
There are real criminals among us, but there are also adversaries of corrupt government activity. If the people in power can intimidate most of the public, they are then able to act unscrupulously more often.
Here is a successful Hollywood producer, Aaron Russo, describing how his friend Nick Rockefeller wants to create a society where everyone is chipped and the people in charge can prevent citizens from having money to even buy food if they don't toe the line.