Minorities, the poor, the elderly are all statistically less likely to maintain an up-to-date ID. And in the case of poorer individuals, most of these IDs cost money and/or require documentation that if you haven't maintained (like birth certificate) costs money as well. In addition the process of getting an ID can be burdensome on those who barely get by (work sometimes multiple jobs, travel time for work, kids, school, etc). While the burden may not seem particularly heavy to those of us who are amongst the more privileged, for less fortunate it is oftentimes enough to simply not bother maintaining the ID.
Thus adding an ID requirement can and studies have shown often is a deterrent to vote for millions of fully legal voters.