Creative agencies are agencies that will take care of all your artwork, design and writing needs, plus often a lot more. If you can find the right creative agency, then that is half the battle. You should put in the hard work during your search so that you can find an agency whose ethos complements yours. This will help lead to many years as a productive partnership.
A creative agency will save you a lot of time and money if they are able to do everything you need, all in one place. This will save your outsourcing your design work to a variety of companies, which can become an administrative nightmare and can really begin to increase your costs.
An all-around creative agency will have experienced designers and writers in-house who are able to provide you with plenty of value for money. They should be able to do anything from designing you a new website to creating artwork for your new print marketing campaign.