The distinctions among Windows user permissions and any set of SQL Server GRANTs seem like unrelated concepts. As often as not, it seems to actually be implemented with pseu
Many of these have been said or are similar to previous answers... With AD integration:
a) I don't have to worry about the users who have access to any given application, I can pass that off to the security guys.
b) I can restrict access at a table by table level based on groups that already exists, as well as forcing standard users to only have the ability to call stored proc's.
c) When a developer leaves my group, we don't have to change all the DB passwords (i.e. if you care about data security...)
d) It's easy to do custom logging based on the user who makes the change. There are other ways to do this, but I'm all about being lazy.
e) Integrates with IIS authentication. If you're already using IE & IIS for your intranet, this just makes life a lot easier.
Note: There are far more reasons not to use it, and I never used it before my present position. Here where everything is lined up in AD already... It's just the easiest time I've ever had with database security.