As Dalford said, it is also handy when using the individual slide metering valves, to open only one slide, or to keep from opening one particular slide.
However, be aware that when you valve out, a slide or multiple slides, when you valve them back in, you have pressure built-up in those hydraulic hoses.
When you open those valves, (unscrewing the valve knobs), you release that pressure & if you do that after closing the slides, there is now no pressure holding those released slides closed.
The slides can possibly shift during travel.
I try to make a habit of opening the valves prior to closing the slides. Then the hydraulic system will cycle thru each slide, in sequence, as in normal operation pressurizing the whole system.
I/E Bedroom, Livingroom Front Slide, then Livingroom Back Slide.