I believe they are for the date that you wrote/posted it! Or you can doodle inside them and make some patterns
I wonder if they were made in China? If so they’re used for writing post codes of the recipient and the sender
This is the first time I saw a vertical type postcard. I’ve mostly been writing on horizontal cards.
If the card is from China, the boxes are for writing the zipcode if you’re mailing within China, since that’s how they format the address there 
I think they are for date, ID or maybe PSÄŚ (In Slovakia we have this number that represent part of city).
The upper boxes are for the post code of the receiver and the lower boxes are for the sender. But those are for domestic mails. There isn’t any use of it when sending abroad.
When I write to receivers who doesn’t live in PRChina, I use these boxes to write date, weather and temperature, or just scrawl a smile to fill them. Or maybe I put some stamps on them because they waste lots of space for writing.
In Ukraine we also have this boxes for zip code, not useful for sending abroad. We have it in the bottom of the place for the address, so I write country there if the name consists of up to 5 letters.
I have these on cards I purchased in China.
I use the little boxes to write the ID 
I’d like to, but there are only 6 boxes, but the ID of China is over CN-33xxxxx. 
I have seen these often on Japanese postcards, and I think they are for Japanese postal codes. But I’ve never seen a card addressed with one, because I’m not in Japan
In Japan and China (especially in Japan) postcards tend to be vertical
The boxes are reserved for writing the zip code
Australian postcards and envelopes often have 4 little boxes like that for Australian post codes. Which in my opinion is a bit silly seeing postcards are frequently purchased by tourists for sending to their home country… 
In Canada, our postcards often have six boxes for the postal code. I’m not sure why there are two sets of boxes on the card you have shown, however.
In PRChina, we are supposed to write the address of ourselves. So the boxes are for the “return address”. I have this set of postcards which produced in PRC, so I am sure about that.
That wouldn’t work here because, if I were to right my return address, my postal code would go at the end. The boxes are at the top.
The standard way to write the address here is:
name surname
street house number
postal code town
But some people, especially older, use also the opposite order:
postal code town
street house number
name surname
It doesn’t really matter for the post either way.
The logic behind the 2nd way of writing is the post actually looks at the address this way - first, they need to know which city to deliver to, then the street and only then the actual person.
That makes sense. Here addresses are read bottom up.
Take this card as a example,
(on domestic mails) we firstly fill the upper boxes with the post code of the receiver.
And then we write the text.
After that, write the address of the receiver. Finally, write the address of ourselves (as return address). Fill the lower boxes with the post code of ourselves.
It’s confusing, isn’t it?
The boxes are designed to write post code in.
So the address (of domestic mails) will be like:
town
street house number
surname name (in Chinese)
*The postal codes are written separately (in those two sets of boxes).
But all of this only works on domestic mails. When sending to other countries, just ignore the boxes at all.