My phone required it.

When purchased, it had an IMEI of all zeroes, and came with a step by step guide to setting the IMEI from my old phone into it.

Apparently that works around phone providers who blacklist or whitelist certain manufacturers.

Can you please expand on your phone? What model is it? It sounds very unusual for the user to be able to change the imei.

A knock-off Umidigi A3 Pro.

It's an awesome Chinese $50 android phone.

Because it hasn't passed all the network tests, most mobile networks would block an entirely random IMEI, so they suggest you type in the IMEI from your old phone (as long as it's 4g - if it isn't, they have a support email address you can contact and they'll give you an IMEI that works in your country)

Even the legit Umidigi A3 Pro's (which cost $80, and have proper certifications) have an 'imei change' tool.

Not for long I'll wager...

I'll wager the opposite. Of course it's not legal anywhere, but I don't expect any customs people to be able to tell that a phone comes with an changeable/unapproved IMEI.