Yes they may have the word 'tights' in the name, but cycling bib tights are cold weather cycling essentials, and should be a staple in every rider's wardrobe. However, when does the ride call for bib tights?
The easiest way to decide if the ride calls for bib tights instead of bib shorts is to simply step out of the door and get a feel for the conditions. If your legs are cold within a few seconds, and a strong temptation to jump in a hot bath takes hold, then it's on with the tights! Offering both warmth and protection from the elements (this varies between models), a good cycling bib tight will allow you to ride in poor conditions, but without worrying too much about the cold. Not just for the colder seasons though, rainy, misty days can always surprise you, so they won't be in the wardrobe long before you need them again.
Most cyclists agree that having a chamois inside the bib tights is best. You can buy them without, but it is much more rewarding in the long run to spend a little more and go for a padded version. If nothing else, it saves having to wear a pair of bib shorts over the top just to get the benefit of their chamois.