Why do I need to hide my phone's IMEI
As @Lighty said, the IMEI is a unique identifier for your phone (not the SIM card though, that would be the IMSI).
You can think of it as an equivalent to a MAC address in Ethernet. The IMEI could be spoofed to impersonate you / your phone. Your phone could get traced in a network using the IMEI (It actually is to maintain your connection). The IMEI is also used to find stolen phones, by comparing it to the IMEI DB of the GSM Alliance, for example.
Thus, to protect your idendity, and limit tracing to legitimate usages, it may well be worth it to not spread the IMEI out in public.
IMEI is like a GUID (Global Unique Identifier), that identifies your unique handset. Your carrier can blacklist your IMEI by instructing the GSM Alliance to do so, so that the mobile can't connect to any networks, usually in the case the handset is lost or stolen.
Your handset's IMEI is sent in the handshake process when connecting to a network, and can be searched in the global database by your carrier to identify the make & model of your device. This is useful as it allows the carrier to send over the correct internet/MMS configuration settings automatically, and may turn off promotional messages your carrier may send promoting the newest device if you already have that device.
You would not want to reveal your IMEI for the following reasons:
- Criminals can use an IMEI to do, well, criminal stuff
- It makes your device vulnerable and can be hacked
Scenarios:
- A device manufacturer discovers that the battery in some devices can spontaneously combust and explode. The device manufacturer offers a replacement process whereupon if you present your IMEI you are eligible for a free replacement device. Unfortunately a criminal got hold of your IMEI and claimed the free device, but as the IMEI has already been claimed, you can't claim a new free handset.
- A hacker knows how much you are worth, and bad news, they have your IMEI. Through a simple SIM-unlocking site they can obtain the make and model of the device, and maybe the operating system version residing on it. They can now proceed to send you some spam/dodgy way of installing spyware onto your device. They can now steal your data and thus, your wealth.