What does "unlocking" your phone mean?

@dmrehe To use it with a different cell company. Either in the US or internationally.

Or to sell it to someone else who wants to do the same.

If you're happy and those doesn't apply, then it probably doesn't matter.

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@Conejo1962 wrote:

Not helpful in one respect.  They OP asked “Why” only.

Another posted thread stated that while unlocked, you can use another carrier...that other carrier may be limited due to LTE bands used by other carriers.  True, you are always limited by the hardware and network.  Some phones have many bands, making them more universally excepted.

Most likely, Sprint and Verizon are not options as well.  Most ATT phones do not have CDMA bands and won’t work on Verizon or Sprint because of the network type.  This is coming to and end as those networks will be progressing to LTE only...

they don,t use SIM cards.  NOT TRUE!   All LTE phones use a SIM card.   Only a basic Verizon phone has no SIM card.   

In addition, your phone, though unlocked by ATT, can only get future updates if connected to the ATT network.   This is true of android phones only.  iPhones are not branded and updates always come from Apple.

 This unlocking offer seems to be a bunch of bull.  For the near 50% of users who buy iPhones, and want to switch or travel, unlocking is not “bull”.  

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@Conejo1962

No one forces you to buy an ATT phone in the first place. My family prefers the variety and lower cost (sometimes) of unlocked androids. My oldest son got the new Razer phone, and I got the oneplus 5t.   But I still own an Att branded phone for Wifi calling when needed.  

Thats my choice, but it in no way negates the benefit of being able to unlock a phone sold by ATT if a customer want to do so.