Updated: Sep 11, 2019
There are more reasons than what I've written below but these are the top ones in my book!
Deals can be better. Sellers are usually motivated by circumstances when selling outside of peak season so they may be more willing to work with buyers.
Focus. The focus shifts from the beautiful yard (which is becoming less beautiful this time of year) to the outside of the home. This time of year (winter months too) is actually a benefit that points our eyes to more important things like the exterior of the home, roof, etc.
Less likely to be in a bidding war. The Spring Frenzy that wore a lot of buyers out is over and typically (there are exceptions, of course) the heightened sense of immediacy is lessened this time of year.
Agents are likely to have more time to spend with you. Fewer listings typically means more time. If your agent still doesn't make adequate time - have a discussion with them!