6. Gives a good understanding of what Christians have traditionally believed, thus (hopefully) avoiding heresy.
5. It can help provide strength in the face of adversity (note many of the stories of martyrs passed down).
4. It is difficult to understand where you are or where you are going without understanding where you've been.
3. It shows that some of the issues and struggles that churches deal with today are not really all that new.
2. It hopefully shows the right and wrong ways that people have dealt with said problems.
1. It's just really entertaining sometimes. Take, for example, the Cadaver Synod of the year 897. (I actually read the link and, even though it's from Wikipedia the facts are straight).
Any readers who feel the need can point out other reasons they can think of. If I get enough (any) submissions, I'll post a revised list with credit to the outstanding thinker(s).