It helps everyone in the company since we had marketing usually being on windows, developers usually being on linux and designers usually being on mac. With Figma anyone could browse the designs and before that (when using Sketch), we had to export designs from Sketch to something like Zeplin just in order for others to view the updated designs. Then designers need to remember to update the exports every time they changed anything.
With the introduction of Figma, we didn't just get the ability for everyone to browse the designs directly, but also make smaller fixes directly in the design together with the designers, remotely or not.
I'm pretty sure that this was the killer feature.
This is a combination of no OS dependence, no installation required, and no license key software. The software being any good is just gravy on top.
Anybody who has ever dealt with IT understands just how much pain this saves them.
Figma offers design, prototyping, developer handover, team collaboration, a.o. capabilities. The basic service is free and the paid tier is fairly priced. All said capabilities are also integrated so you don't have to use plugins or switch between apps. When working with Sketch, I use Sketch for design, Marvel/InVision/Framer for prototyping, InVision/Zeplin for developer handover, and InVision Freehand/Miro for collaboration. This means dealing with multiple licenses, plugins, and copy-pasting assets.
I also worked with a mid-sized company that was paying around EUR 20,000 just for their InVision license (developer inspect is expensive). With Figma you get most of these services (good enough) for a single price.
So money certainly plays a role but Figma's success goes beyond just the pricing model.