Not really sure why that advice was being given. It is not best practice, not part of the official docs and not necessary. Perhaps the poster was suggesting it to reduce I/O load (copying lots of data, Syncthing reading that data at the same time).
My advice is: Should you be using a program designed for automatic, continuous file synchronisation? If not, perhaps Syncthing is not for you. If you want a backup program or a system for copying files from A to B but not doing this, that, the other, cherry-picking this, ignoring permissions on a full Moon, feeding excluded folder settings from a shell script more complex than orbital mechanics, then please, think things through first.
If Syncthing does seem like the best tool for your use case: welcome! Take a backup of your data because now you have multiple sources for data corruption, deletion, loss, plus the possibility of a Syncthing bug.
Use the all of the defaults, for the love of all that is good, at least until you’re familiar with how Syncthing works: its behaviours, how it discovers, connects, detects and communicates changes and syncs files; how long things take, what resources it consumes. If you don’t mind tinkering, mucking about, diagnosing yourself, then sure: go hog wild and change 50 settings on day one.
Until you are an experienced Syncthing user, don’t second guess it. Just let it do its job because it is great at doing its job.
The defaults are great, they make for a very resilient system for finding devices, figuring out how to punch through firewalls and NAT and using relays if need be, when and how to detect changes to files and what’s best to choose for the speed of your devices and networks.
Don’t expect a device to connect in 2ms and instantly copy all data because a watched pot never syncs. Sometimes it might take a little while for a device to attempt connection. Just leave it alone for a few minutes! For a set of devices to exchange lists of file changes, to read file from storage, encrypt it, send it across the network, and then undo all that at the other side has overhead, it takes time. You will not get 1Gbps throughput between your Raspberry Pi Zero and your Pentium III 733MHz computer, however fast your network is.
If you have a problem, search the forums first. Use the tools as your disposal to solve the problem: what are the logs on all devices saying? What are the device/folder status panes saying? What has changed since it last worked?
If you do ask for help, help the project folks first. Tell them your Syncthing versions, system specs, network speeds, post Syncthing log files, screenshots of device/folder status, tell them what you have tried, why you think it should not be acting as it should, and please keep the post simple.