If your law department has outside counsel guidelines (See my post of Aug. 1, 2006.), you will find that many law firms negotiate revisions or supplements to your terms. You need to keep track of those variations in a central database.
Furthermore, from time to time you may negotiate special fee or staffing arrangements with law firms for particular matters. Again, you need to keep track of those arrangements in one place.
If you do not maintain a repository of firm-specific terms, you and other people in the department later on will not be able to recall the terms you have reached nor will you be able to look back after several alterations have occurred and perhaps adjust your outside counsel guidelines (or responses to engagement letters) in light of your experience negotiating them (See my post of May 19, 2006 regarding law-firm engagement letters.).