Really there's nothing outside of the ISA being open and freely extensible for anyone who wants to do so. The ISA itself is similar to ARM, MIPS, or other RISC ISAs. If you wanted to build a custom microcontroller for some purpose it would be cheaper to use RISC-V since it doesn't have licensing costs. Most companies aren't doing this though and just by OTS components that work with their codebase. If you wanted to create some dedicated hardware paths for computationally expensive operations, the RISC-V ISA lets you add those in. You'd need your own compilers to generate those instructions or to write the assembly code yourself, but it gives users that flexibility.
Otherwise there's nothing inherently magical about RISC-V that would make it better than ARM in some performance metrics.