A marketing company should not only carry out specific marketing task but should analyse your current business situation and create a marketing plan based on your requirements and budget. They will often become more of a consultant to your business than just someone that looks after your website and online and offline promotions. These people are the experts and therefore will be aware of any recent advances in the marketing world that may assist your growth.
Marketing is all about being proactive, it should be something that is on going and monitored closely. Any body spending money on marketing needs to be able to try and establish how successful it is so they can work out if they should continue to invest down that particular avenue.
Usually it is quite straightforward to monitor the performance of an online marketing campaign, offline marketing such as leaflet dropping or TV / Radio ads may require a little more pre planning to be allow you to monitor it.