Respectfully, this question has been answered numerous times, and asking additional poster to answer it again won't give new answers.
The argument is that different states have had different requirements for issuing driver's licenses and state identification cards. After 9/11, the terrorists were found to have multiple state IDs and DLs. The Federal Government, Democrats and Republicans, decided that having a measure of uniformity in requirements for getting/verifying state IDs would help cut down on issuance of multiple IDS or fraudulent IDs, and would thereby make flying safer by having more accurate identification of those boarding airplanes.
Those Democrats and Republicans didn't really have an ability to coerce the state governments into whatever standards they wanted to impose for issuance of those IDs, so they coerced the states into providing the Federally-sanctioned Real IDs, with the new uniform "secure" guarantees, in order to overcome "reasonable restrictions" placed on certain rights, such as the right to travel domestically by airline - a timely and legitimate concern in the early to mid 2000's. Once citizens of the states had to have Real ID to travel, the states had to meet those standards in the issuance of those IDs for their citizens to travel. Federal Government coercion of State Government, who would have thought it, right?
With this history, (the notion of Real ID somehow being more "reliable," "safer," "more secure," "harder to forge," or whatever framework you wish to give it), the government is now trying to use those Real ID justifications for flight and apply them to the voting booth. After all, if Real ID makes it more likely you are who you say you are when boarding a plane, it follows that it makes it more likely you are who you say you are when entering the voting booth, thereby decreasing chance of fraudulent ballots and, presumably, increasing trust in the electoral system.
There is it. That simple.
Now, I have my own doubts about the practical impact of this all, not to mention the true motivations of the proponents of this action, and it is apparent you do as well. I suspect we agree on the lack of necessity of requiring Real ID to secure elections, if not the utter futility of the entire exercise. Regardless, I gather that is the argument: Better IDs make for less likelihood of fraudulent votes, which is good, just like they make less likelihood of terrorists getting on to airplanes, which is also good.
I don't think anyone saw Real ID to be a future impediment to access to the ballot box, but here we are. I suspect this is just the beginning of all of our government's "good" for us on the Real ID front. Once one gives the government the ability to control one's life, government rarely willingly and freely gives back the tool of that control.
Cheers,
RW