Agreed. If you're going on a business trip or conference this might not be possible.

Photo by couriermail.com.au
However, if you're planning your own little getaway, this could save you some moolah (well, the savings are generally on either one - the flight or the accommodation).
It's important to note that peak and off-peak seasons can vary considerably depending on the country and so some research is required.
Speaking of research, I've also Googled "School Holidays [location]" and "Public Holidays [location], for example, to ensure I get to experience the sights without constantly being surrounded by a swarm of fellow tourists :)
Granted you might face some rains and such when travelling during the off-peak season, but if you're willing grab your raincoat and to chalk it up as part of an adventure, that's exactly what you'll get - A lovely adventure with tonnes of laughable moments and stories to share! - At least I did! :)