Every website page ends somewhere. Does your visitor know what to do there? Often not. Copy and photos look good, but the only next step is a forgotten Contact menu item. Interested visitors leave because they find no clear action.
The call to action, or CTA, bridges interest and action through a button or link. It is the cheapest website element to improve: no redesign or extra budget, just better choices of wording and placement. Figures show those choices matter surprisingly much.