It has been said that implementing Salesforce is a Journey and I must agree. Working with the platform in an enterprise starts small, maybe in one department, but quickly grows to other areas and capabilities. As demand grows in these other areas, sometimes outside your direct area of expertise, you need more information from people who have experience with the platform in those disciplines. Vendors can be your answer to this requirement.
Vendors come from all walks of life and have varied experiences which you can use to your advantage. You can make use of various SIs and AppExchange partners. Remember, if you need a capability, make sure you look at the AppExchange first, someone else has more than likely had that same need or a similar need before. Vendors, such as SIs, provide the experiences in your vertical and beyond to help you navigate the subject area in ways you have possibly not considered. More importantly, the right vendor can challenge your current way of thinking and possibly help you find a better way to complete the task.
Another advantage to having a solid vendor relationship, is lowering the pain factor. When you start into a new initiative, there are pitfalls you will run into. The vendor, if experienced, will know of these pitfalls and help you navigate around them. Having internal expertise on your team is important, but having the outside view, from someone who has already been there, is priceless.
Vendors can be expensive and their flexibility in this area will be limited. But one thing you must always keep in mind, is you are not necessarily paying for the resource to hit the button. You are paying for the experience these companies have had and their ability to translate your requirements into the solution you require while lessening the pain.
Keep in mind as you go through the journey, it is not about having or not having knowledgeable staff who know and understand Salesforce. The Salesforce ecosystem is full of other resources that have more experience than you most likely have in-house that can help make you more successful with less pain.