Simply said, Tableau presents data visualization in a way that is easier for new users to implement and outsiders to understand. You don't have to have a lot of skills to get started, in fact, someone with almost no experience could pick it up within a few hours of tinkering with it. On the flip side, for those who take the time to really learn it; learn the calculations, and what others have done, you can begin to do some great things with the software.
To get into the more advanced calculations, there is a learning curve and it becomes a lot steeper when there are more specific tasks. I would say there are "limitations", but there almost always a way around them. In other words, if you want to do something specific, but can't figure it out, there's typically an alternate way to do it. Although you can complete, you ask, "Why couldn't it have just been done this way?".
If you manage large amounts of data and have to present that data to others, Tableau is the way to go. It makes it easy for you to manipulate the data in an attractive way. Then, for those who wish to view it can do so for free with the Tableau Reader.
There was not an easy way to visualize data. Sure, excel does do the trick, but when you're dealing with months and months of sales data over time for multiple campaigns you can't put all of that info in a single excel file. It simply doesn't work. Not to mention the added time to create graphs, charts, tables, etc. Tableau does it all in one. The benefit is doing more in less time.