There's several reasons for trimming cases. When you roll crimp rimmed handgun cases, the length has to be uniform, else kaka.
For rifle, one of the accuracy secrets is to have a nice squared mouth on your cases. This will help release the bullet straight, much like a crown on a barrel. I never trim to SAAMI specs (recommended trim length in most reloading manuals) but I do take just enough off to square the mouth.
As was mentioned before, if you buy new brass, it always comes with non-uniform length and a raggady mouth. A must for trimming.
I have an old Pacific trimmer that has been a work horse. It came with a removable handle so I can turn the crank if I only have a few cases or I can chuck it up in an electric drill and go like hell. The Pacific uses standard single stage press shell holders to secure the head and a pilot in the cutter head for specific calibers. I believe Hornady now sells this trimmer.
What ever you buy, be sure to see what accessories the trimmer comes with or what extra stuff you have to buy. Some of the pilot kits cost more that the trimmer. It's also nice to be able to motorize it too.