The reason is that we often need to comment OUT code during development. In C++, the 4 choices are: (1) #ifdef, (2) bool dothis = false; if (dothis) { ... }, (3) /* * /, (4) // ...
Among them, I would say the best and least-confusing way to comment out a block of code is /* * /. #ifdef's are very confusing especially if you have multiple of them; // needs to be applied to every line so only works in small scale; if (dothis) doesn't work across multiple functions, and at a glance it is harder to discern whether this is a "comment-out", or a legit condition.
However, by using /* * / in your comments, you eliminate the possibility of commenting out blocks of code that contains /* * / comments. The problem, of course, is due to how the opening and closing symbols are matched.