There's no direct train between Stratford-upon-Avon and either port. You'll be changing somewhere along the way, Leamington Spa, Oxford, Reading, that kind of thing, and once you add the waiting around it can easily run past four hours. That's before you've dragged suitcases up and down a few platforms.
A transfer skips all of that. Fixed time, one car, door to door, no worrying about a missed connection. It's often not much more than what you'd pay for a couple of weeks of port parking anyway, if driving yourself was the alternative.
What's Included
Fare agreed before you travel. A driver for the whole journey rather than a shuffle of different cars. Meet and greet at the terminal on the way home, with your driver waiting at the agreed point. Help with the bags, both ends.