For billing, mainly.
The phone number in 3GPP protocols is the IMSI = international mobile subscriber identity, which identifies a user on the phone network. You swap out your SIM card, and you swap out your IMSI. You roam abroad, and the foreign network knows who to send the bill to.
The alternative would be to keep an international database of IMEIs (device IDs) like CDMA networks did. Using SIM cards with IMSIs ended up being more consumer-friendly.