It's a cargo flight, after all.
Many/most passenger airplanes handle cargo business ordinarily .
(Amtrak who thinks it's an airline, took it to extremes; ordering hundreds of baggage cars specifically for freight, you had Amtrak trains with 25 baggage cars and 5 coaches.
Their freight railroad hosts told them to knock it off.)
However, the COVID-19 crisis has caused great shifts in the economy - largely ending the passenger business, but a veritable "gold rush" in the air freight business , particularly for emergency supplies that cannot wait for the slow container ship.
Your particular flight is from China, which as you know is where a tremendous amount of material is manufactured.
And it's to Europe, which is pretty much the worst case for container-ship shipping times (must either transit Panama, Suez, or best case, a double transload for a rail crossing of North America).
As such, the flight you mention is surely kept plenty busy by that traffic.
They can use almost the entire baggage hold for freight, and some airlines are pulling out seats so they can load cargo in the passenger area.
This is perfectly legitimate; in fact many airliners are made as "Combis" that either convert quickly, or have a cargo section aft of the passenger section.
travel.stackexchange.com/users/55372/Harper - Reinstate Monica
China Eastern airlines are used to haul face masks from China to Czech Republic.
Czech national radio website mentions China Eastern delivery from March.
I also found and an online newspaper article from 10th of April referring to China Eastern delivering face masks and Airbus 330–243 is specifically mentioned in that article.
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Few countries have actually “closed” their borders.
The Czech restrictions seem to be among the most extensive, even after being relaxed somewhat , but do leave the possibility of travel for very limited reasons open.
In particular, the Czech Republic did not, to my knowledge, close their airports and interrupt all flights into the country (which some other countries did do).
As to why that particular flight did happen, it's difficult to know for certain.
There have been many discussion of nearly empty flights being operated for legal reasons (safeguarding slots and the like) but I am not sure that's a factor here.
Freight is certainly a strong contender and Covid-19 actually created some new demand in that area as protective equipment, reagents, etc.
are often sourced from outside Europe and especially China.
Finally, even if travel is limited to a handful of diplomats and experts, the occasional aid worker, and a few stranded nationals/residents trying to come back home, if you want to maintain safe distance inside a plane, it will fill up very quickly.
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Some combination of: They can (i.e., flights are permitted by the Czech authorities); and There's money to be made now (from carrying either freight and/or passengers); and/or they want to keep their routes, and gates, and airport arrival and departure slots (which can all be at risk if they're not used); and/or they wish to pre-position their service to be already up and running as things loosen up in the transportation world.
The actual internal reasons and decision-making behind #2 are apt to be invisible to outsiders.
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Czech Republic has been keeping their borders shut for more than a month now Incorrect.
A fair amount of nationals of over 30 countries can still enter Czech Republic (see https://www.iatatravelcentre.com/international-travel-document-news/1580226297.htm ( mirror ), e.g. people under age 18 or spouses of nationals of Czechia).
And there are over 1.4 billion people who can still enter China.
More generally there are still quite a few lines in operation (e.g. see this samchui.com
post ( mirror )), sometimes with reduced frequency.
travel.stackexchange.com/users/1810/Franck Dernoncourt
The Czech Republic has been keeping their borders shut for more than a month now and likewise China closed their borders to all foreigners back in March. However mysteriously there is a daily flight between Shanghai and Prague:  MU707 . What is the purpose of this flight and who is making use of it?  As pointed out by @Relaxed this is the only non-chartered non-cargo flight that landed in Prague airport today, so there must be something special about it.
Why does China Eastern airlines still fly daily to Prague?
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<airlines><covid-19><prg>
travel.stackexchange.com/questions/156815
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This is a cargo flight. Reasons for still flying include: flights are permitted by the Czech authorities; carrying either freight and/or passengers; keeping their routes,  gates, and airport arrival and departure slots (which can all be at risk if they're not used); and/or they wish to pre-position their service to be already up and running as things loosen up in the transportation world. Few countries have completely closed their borders. Borders have only actually been closed in a small number of countries and cargo flights remain operational.
Flights are still operating for freight, such as face mask shipments. Borders have only actually been closed in a few countries.