Every wall, floor and ceiling is not exactly datum. The untrained naked eye cannot spot the slants, bends and wave. This makes installing cabinets very difficult. The following are problems you might face with an uneven floor.
Built-in cabinets are usually built on site except for unfinished cabinet bodies. Laminates are glued on site, adjustments are made on-site. Backs of cabinets and other hidden sections remain unfinished to save costs. The finished cabinets look snug against walls and ceilings.