Instead of cryptocurrencies I was going to just say crypto, but I did want to close out what I mentioned a couple years ago before the (FOMO retail investor fueled) bitcoin run up and inevitable sell off; hopefully it kept someone from losing. On to cryptography.
Factoring large numbers is largely used in encryption. As a small example the number 6 has 1, 2, 3 and 6 as "some" factors, BUT something to remember from high school algebra is "i", the square root of -1. [(i)^2 = -1]
if we multiply (-2i)(3i) or (i)(-6i), we also get 6. That gives computers more to think about.