The cation exchange capacity (CEC) shows how well a soil can hold onto and store cations, so a soil with a high CEC would be able to hold more nutrients. A soil with low CEC for example would not only be missing some important nutrients but would also not be able to hold onto nutrients as well as a soil with a higher CEC. So even adding nutrients to a soil with a low CEC would not be very effective in improving fertility unless soil conditioning was applied that would help improve the CEC as well. Soils with a lot of cations can also hold onto water better since water is a polar molecule and is therefore attracted to the positively charged cations (hydrogen bonding).
Plants that grow in soils with a high CEC value do not have to spend as much energy looking for minerals and water and therefore are able to devote it towards growth. Microorganisms that are essential for good soil health also thrive in these conditions.
However, a soil with a high CEC can also hold more of the acidic hydrogen cations (H+). So when the soil pH of a soil with a high CEC needs to be adjusted it often acts as a buffer and therefore more acid or base is need to change the pH.