Should the exit node be owned or compromised by an attacker the attacker will have it easier to analyzing your traffic (using your behavior, writing style, ...) . Depending on how you use Tor it might in certain circumstances make it possible to at least guess who you are.
If the traffic you produce is spread over more exits an attacker has to control or analyze more total traffic. Tor basically works by hiding you in the crowd of its users.
For example when an attacker sees the user visits a certain web forum every day and the traffic you produce when connecting to that forum is unique enough an attacker might use this to create a profile. Even more if you use Tor a lot for various different things, always using the same exit. If all that behavior gets recorded an attacker might be able to reason about the produced traffic. How well that works depends on many factors, how and what for you use Tor and what the attacker's intentions are.