Why were poll taxes created?
To suppress the vote of black people in the Southern US, the majority of whom had just been freed from slavery.
In a democracy, voting is important. It's how the Will of the People is transformed into elected representatives that then work in government to bring about the kinds of things we want to see done: the country made strong and secure, opportunities for all, clean water, air, and food, and all the other things we want government to do for us.
There are two kinds of people who try to influence the vote - those that try to get people to the voting booths, regardless of colour or creed, and those that try to prevent certain sections of society from being able to vote.
As to why someone would want to suppress the vote - if you were running for an office and you knew that a segment of the population, let's say people with the last name of Smith, were overwhelmingly against you and were going to vote for your opponent, you'd probably hope that they didn't vote. You could go one step further and try to prevent them from voting - let's say you passed a law that said that everyone with the last name of Smith had to pay a tax in order to register to vote - that might keep the Smiths from voting and helping you win the election.
And that was the point of the poll tax - to keep people away from the ballot box.
After the US Civil War, in which slavery was abolished, there was a period of Reconstruction 1863-1877) where Northern government, armies, bureaucrats, and others worked to help get the South back on its feet and to ensure slavery was indeed ended. And then the US ratified the 15th Amendment, in 1870, which stated that the right to vote could not be denied due to race, colour, or previous servitude (it didn't matter if you were once a slave - you could now vote).
After the North left, Southern politicians began to discriminate against blacks and segregate them under the mantra of "separate but equal" - which really meant absolutely separate but never equal.
As part of this segregation and discrimination, the Jim Crow laws were passed - laws that systematically took away rights from black people. One of the ways they did this was to pass Poll Tax laws - which stated that in order to register to vote, you needed to pay a tax. Which sounds innocent enough, until you remember that at that time, black people were, in general, quite poor and couldn't afford to pay the tax. Meaning they couldn't vote, so they couldn't stop more laws being passed against them. It was a way to deny black people from voting without actually saying that black people couldn't vote.
It wasn't until the passage of the 24th Amendment, in 1964, that poll taxes were abolished - but that was only for Federal elections. It took a reinterpretation of the 14th Amendment in 1966, under the Equal Protections Clause, to finally put to rest poll taxes.
As an aside, as modern-day politicians talk about making sure the vote is secure and as a result are demanding people provide a driver's license as voter ID, and we also hear about license offices closing in poor and underserved neighbourhoods, and fees going up to secure those driver's licenses, ask yourself - are those politicians really wanting to make sure the vote is secure? Or is this just a new way to create a poll tax?