Sketch was the market leader in ui design tools. Before that, it was photoshop. Only a step away from using something like blender or after efffects tbh. It was also mac only. And desktop only. It turns out design is one of those things that people like to see, and is not insrcrutable like code. So stakeholders asked to see your ui and you would send them... this file. They would then have to download the application, and deal with all the joys that come with different platforms, asking how to install the thing, security complians, licenses, etc.

Figma came in with a web app that made designing and sharing as easy as sending a link. They also had... let's say creatively viral approaches to licensing where anyone that edited a file automatically got added as a seat. But unlike those desktop applications, you could also leave notes - that's editing! So it wasn't just for designers the way photoshop or sketch was. Now instead of your team of 2-3 designers, it's like half the company. It's beautiful in a way that the latter group is way more numerous and uses <1% of the software features, yet gets charged just as much. Beautiful. And lucrative.

Anyways, they're similar because Figma made a web-based ui tool, and the base model for the workflow was already established by Sketch, so their fundamentals are very very similar.