Single-density foam chamois use the same firmness of padding throughout, so they don't have the firmer support under your sit bones or softer relief in the perineal area that higher-end pads offer. They're the most basic option you'll find, typically in entry-level and budget cycling pants.
For short rides under an hour, indoor spin classes, or casual errands on your bike, a single-density pad can work just fine. They provide a baseline layer of cushioning between you and the saddle. However, if you're planning rides longer than two hours, the lack of targeted pressure relief often leads to discomfort—your sit bones and sensitive areas all get the same level of support, which isn't how your body is shaped.
If budget is tight and your rides are short, single-density is acceptable. But if you're regularly riding over an hour, investing in multi-density or anatomical pads is well worth it for the comfort difference.