Why is the storage format of SUPI a string and not a pure sequence of numbers?

SUPI should be a 15-digit number like IMSI, why do you add “IMSI-” in front of SUPI in the free5gc code?

In 3GPP spec R15 TS:29.503, we need to add “IMSI-” in front of SUPI.

Below is the screenshot of the spec.