There is a point in everyones ancestry research which requires an expert. This can be a family member or genealogist. Sometimes genealogists are hired not just because a person can't find a specific person, but because a person wants them to put together a family history as a whole. Many people don't have the time it takes to research their families ancestors. I have talked to cousins who I would consider very knowledgeable in my family line. Their is always going to be someone who is more resourceful or knowledgeable than you. If you have exhausted all family options and you have to get your research done, then hire a genealogist. It is possible, they can't even help.
Location is key. With the age of the internet, many files are being put online. But, there are still millions that are not. You will have family that was in a location inaccessible to you. Genealogist are their to help you in these situations. In most cases it is going to be cheaper to hire a genealogist, than to fly to the location you need to be in. You are looking at hotel costs and rental car costs. Genealogists are there for cases where the cost to hire them is a better option than your travel costs. Some like the adventure of traveling to find ancestors. If you have the money for that, then more power to you. Otherwise, hire a genealogist or contact a family member that will do the research in the area you need.