This is a question many companies ask when it comes to looking at their marketing strategy. Marketing covers a wide range of techniques and avenues some of which can be done yourselves and others of which should be done professionally to get the best out of your budget.
Google Adwords is an example of online marketing and many companies chose to undertake this themselves rather than outsource it. It is relatively easy to create an Adwords campaign, but creating one that works and gets the most out of your budget is something different all together.
When setting up a campaign you need to be sure to check times your adverts run, keywords you target and match types along with negative keywords, locations and the content within your ad etc. If these are set up incorrectly then you can end up paying for clicks that are never going to convert in to enquiries or sales, essentially wasting budget.