A recording console has two types of signal source. One is from the microphone and line inputs. The other is from the already-recorded signals on the multitrack tape. To record overdubs, these must be mixed together for the monitor mix and headphone foldback.
A split monitoring console has a number of input channels, plus a number of monitor channels with reduced facilities.
Since a simple monitor mix is often all that is necessary, the monitor section of a split console will typically offer only level and pan controls. PFL is handy too.
Although the split monitoring console is now considered old-fashioned, there are still such consoles in day-to-day use. The concept is still valid and we may see a return to split monitoring consoles in the future.
Also, an understanding of split monitoring helps make it clear why current consoles use the alternative inline monitoring.