I tend to be pretty lazy when it comes to compiling my code. After I'm done knocking out the source, I want to make sure it compiles right away, and start testing it. However, taking the 10 seconds to add warning flags to the compile command could save me, and you, countless headaches trying to track down a bug that the compiler would have been able to point out right away if given the proper flags. The following example, unfortunately, is immune to compiler warnings, but it illustrates a basic error that compiler warnings would try to prevent you from making.
I was writing an application that dealt with unsigned integers throughout the entire program. At some point, I needed to convert those unsigned integer values into std::strings via a helper-function that I created. The values were read in via a binary file, and stored in unsigned integers. The deadline was approaching, and I didn't think about what would happen if there was a negative value in the binary file (perfectly legal, though) The result? The compiler silently converted the int that I retrieved from file to the unsigned int it was being stored in, thus mangling the sign value on my data. Luckily for me, I had been programming on a daily basis for many hours for about the past 5 days, so my brain was warmed up and I caught the error rather quickly. However, it isn't hard to imagine how something as minute as this could remain hidden for a long time, delaying the completion of your project, and driving you farther into insanity. Again, this particular mishap is unfortunately immune to warnings, but from this you can see how something of this sort could be avoided by enabling proper warnings.
There are many other error flags, please check the docs for your compiler to see what they are, but it should be a goal to not only use these flags at all times, but also to have your code compile with the strictest of warning flags in use. If you are receiving a compiler warning, there is a good bet you have a problem with your design, implementation, or maybe both. The above scenario would really be classified as design/implementation, depending on how you look at it, but really it was just a case of carelessness.
Enable compiler warnings with the needed flags. Get rid of them with better code.