edm and CS-Studio connect once and then keep that connection.
This is useful for two reasons.
One is the performance gain respectively reduced load on network and IOCs as you already mentioned.
The other is that we're in a distributed environment.
Your display that has a text entry field or a slider might not be the only tools that's writing to those PVs.
In fact the IOC might just change the PV on its own.
Or you write "105", but the IOC clamps the value at "100".
This means that you always need to monitor a PV that you write.
You move the slider or enter a new value into a text entry field
-> Your tool writes to the PV.
Meanwhile, the PV can be changed by somebody else
-> Your tool needs to show that value in the text entry field, or move the slider to the received value.