Good question to ask.
In short, the reading I've done lends to the notion that if you buy fuel from one of the major gas providers, the additives that they have in their fuel blends are enough to keep your engine running clean. However, not everyone buys from the big guys. So, lower grade fuels can cause buildup in the engine. When used sparingly, Seafoam (or the like) can remove deposits. The concern is what this or other additives do to your engine other than removing the 'harmful' deposits. Some suggest that prolonged use will damage the engine's soft petroleum based components. Basically, a bottle of the cleaner might not be mixed proportionately as prescribed and do some harm.
Here's a link to a related item - www.skepdic.com/comments/slickcom.html
I believe if you do a search on the web, you'll find other articles of a similar kind. Most suggest additives are 'snake oil' and not worth the investment. If that's what you conclude (as I did), try running the 'big guy's' fuel blends for a few tankfulls and see if your MPGs/Performance returns to norms. If not, a professional cleaning, while more expensive, might be a good alternative. Good luck!!