Because even a long (64-bit int) is too small :)
A long can hold 2^64-1 = 1.84E19
A double can hold 1.79E308
Double does some black magic with an exponent, and can hold absolutely massive numbers!
Double also has some situations that it defines as “infinity”, a concept that does not exist in long as far as I know (?)
Because paying a few grand a year for a certificate somehow makes your software more trustworthy