Spacers are 5" wide (and 3/4" thick, like all our products). They are meant to slide in between spaces, like a 3/4" book on a bookshelf. They can be added to your space for functional or aesthetic reasons.
A functional reason to add a spacer would be to create space where it is needed. For example, IKEA recommends adding space around your dishwasher for a moisture/humidity barrier. You can add a spacer on either side, so your IKEA cabinets have an added layer of protection.
A design reason to add spacers would be to match the style of another cabinet. If you've got a short wall cabinet flanked by two taller wall cabinets, the tall cabinets will get our 3/4" thick side panels to cover their exposed sides. Some folks add spacers on the base cabinets below to create the appearance of matching panels below. With the spacers on the base cabinets, the wall and base cabinets line up and are exactly symmetrical, instead of the wall cabinets taking up an extra 1.5" of space.
Spacers can also be a fun way to break up your kitchen cabinets with a pop of a different color or material. One combination we love is a white high-gloss acrylic cabinet with wood spacers and wood side panels.
Above: These two cabinets have a wood spacer installed in between them, creating an air gap and a fun pop of color visible from the front. Once the countertop is installed, no one will be able to tell the spacer isn't a full panel.