What’s the whole deal with swapping chains ? And some people keep the chain and put it in again after a certain amount of time?
The idea is to replace a chain before it starts wearing the sprocket and chainring. Chains are generally relatively cheap compared to chainrings, so this usually makes financial sense. Sprockets can range from dirt cheap (Sturmey and Shimano hub gear sprockets), through moderately expensive (Rohloff) to very expensive (high end 11 speed derailleur cassettes).
It can make sense to run several chains in alternance on a single set of sprockets/chainrings. For example:
start with Chain 1, take this off after 300 - 800 kms, clean sprockets/chainrings and fit Chain 2.
after another 300 - 800 kms, remove Chain 2, clean sprockets/chainrings and fit Chain 3.
clean and relubricate Chains 1 and 2.
Doing this properly is relatively time consuming.
after another 300 - 800 kms, remove Chain 3, clean sprockets/chainrings and fit Chain 2.
I find this useful for derailleur bikes (only 1 left now) and Bromptons, where I can't fit Chaingliders.
Another approach (that I haven't tried) would be to use chains, chainrings and sprockets that are supposed to last much longer than standard components. KMC make chain, chainring and sprocket sets that are supposed to last 10,000 kms, but, as for the Chainglider, only a few combinations are possible. These are probably not compatible with a Chainglider, but if the durability claims are true that might not matter.
https://www.kmcchain.eu/10000km