Training only on the bike can bring about structural weaknesses. If we're lucky, these can be inadvertently addressed by activities we chose to do off the bike. If not, they require the eye of a good coach to diagnose and address. Either way, the offseason is the best time address them. Coach Matheny also mentioned that unless you're a pro athlete with virtually unlimited training time, you're usually better served by addressing weaknesses for at least a portion of your off-season, and how it's easy to get addicted to putting in big miles and end up not ever being recovered enough to do quality intensity workouts to really increase your performance.
If you're like most working cyclists, you're winter workouts are confined to an indoor, stationary trainer, which can lead to mental fatigue over time. Coach Daniel Matheny has some great tips for building fitness by interjecting variety into your workouts to keep you fresh and make you stronger. You don't need to grow bored in order gain fitness.