What pinguino wrote is totally correct.
Below is a chart with setting 3m/6m/10m and the same chart with depth setting 1m/3m/6m. Detail here is the tidal difference here is about 2m. This means the dark blue color I cannot sail and the light blue color I can when the tide is right.
The pittfall is the levels need to be included in the chart data. I have some
charts with a fine level display (each 0,5m) and some with only the minimum levels like 1m, 3m, etc.. So if I try to select 1,5m the display depends on the available data.
You'll have to find out what works best for you (your
boat and your preferences) and the
charts you are working with.
PS another pittfall, the depths you want to show may differ between different charts. Sometimes you may want to change the setting to prevent "all white" maps.