Don't know in this case, but some other countries ask for it to deduce your travel history (any current passport itself goes to them, with color copies of every page of old passports). Qatar wanted it because I shared name with somebody born 40 years before me.

Interesting. Just out of curiosity: did you naturalize as a Qatari (didn’t even know it was possible!), or was it a visa-related thing?

I just looked it up. I was dating a Qatari and didn't even dare to explore naturalization. The process is insane. As a man, you can't get citizenship through marriage (only women get that). So, as a guy I would have to live there for 25 continuous years with little time outside the country. Add on that they require you to be rich, Muslim and speak Gulf Arabic at native level. Also, you have to give up your birth citizenship (not always possible) and they can revoke your citizenship on a whim.

Qatar is a fascinating country, though.

No, I can't ever. I was an immigrant worker there, and Qatar Immigration wanted all this before they give me work visa aka residence permit.

Another reason is that the UK forbids the child to have different names on their japanese passport and their UK passport, which is often (almost always?) the case