And no,I wouldn't learn it nowadays. It's more like a blast from the past. If you mean the general appearance with "looks pretty lame", yea well it's old. Grey was trendy once and afair it is a java app:)
Also, not to mix things up: This is a concept mapping tool. Which means, nodes, edges, labels, with no enforced structure. Unlike mind maps (what cmaps.io seems to be), which have a tree structure.
What I'm saying is, how does it benefit me? If it's basically just a graph with informational nodes at the end, what do I do with that?
I can vaguely imagine that if you set it up right you could kind of quiz it like a database or something, but I don't know if it supports that or similar. Essentially I'm clueless, if you could point me to part of the website that sells the idea to me, that would be really good.
I can think of two usecases, research and creativity.
In research (not scientific research necessarily), you can fight the "I don't know what I don't know" problem. For example, all the subcategories of math, write them down as nodes as you discover them, connect with edges when they are related somehow. Put a label to the edge, if necessary.
The nice thing (imo) compared to mind maps is the freeform factor. You can have orphan nodes sitting on the edge, the edges don't need to be directed, you can have cycles.
In creative manners, you can for example have elements of a story, characters, places, events, and connect them as you see fit. Being so freeform, you don't get distracted by being forced to classify (as in, define the position inside the tree structure). Note down open questions, contradictions as orphan nodes in the vicinity.