The intent of the two customization Zones are to let you turn your City District into the type of District that you want for that planet, whether it's an Industrial District (Industry + Industry, or Factory + Foundry), Research, Unity, Trade, or some weird combination of them.
They also let us restrict buildings to those slots if we want, or allow special Zones based on planetary features, civics, or other player choices.
For me, Zones make the game worse when I have <8 planets, and slightly worse when I have 8+ planets and can mostly ignore zones.
Zones could be used to make the generic districts feel special*
Zones instead merge districts to make City Districts more generic**
*Special: better, greater, or otherwise different from what is usual.
**Generic: Not Specific.
If I want more minerals I build a mining district,
If I want more food I build an agriculture district,
If I want more energy I build a generator district,
If I want more housing I build a City District,
If I want more research I build a research district,
If I want more trade I build a trade district,
If I want more unity I build a unity district,
If I want more CG I build a factory or industry district,
If I want more Alloys I build a forge or industry district,
If I want more of: (Research+Unity)+(CG+Alloys)/Soldiers/Trade+Housing... I build a City District,
with a job ratio of (20:20:20:40) : (50:50)
(amusingly it's 90 jobs total and not 100 for research zones, probably as 100 doesn't divide neatly by 3 so 10% of jobs are just missing)
That's far, far too much to try to squeeze into a single general-purpose district.