The cobber easily fits in your hand with your index finger at the tip of the tool where you apply pressure to your masking tape to get a 100% seal on your tape to stop the paint from seeping under your tape. The other method used to seal your tape is with your finger which gives you only about a 50% seal on your masking tape, this is how the paint bleeds.
-Areas we use the cobber after applying masking tape & before we start any painting-
Walls & ceilings
Inside corners before painting accent colors
Basically every surface you tape you must have a 100% seal on your painters tape before you start painting or you will have a mess to clean-up or touch-up from paint bleed.