Johnny:
1. They haven't got access to any lower fares than those that are available directly from train operating companies or via the National Rail Enquiries website:
https://www.nationalrail.co.uk/ ;
2. They charge a booking fee, meaning that you actually pay MORE by booking through The Trainline ;
3. If you find that you've got a problem with your booking, you can't simply go to a station booking office to get things sorted out. (They'll tell you to contact The Trainline, which isn't much help, say, if the train that you want to be on is leaving in a few minutes time) ;
4. They're notorious for messing things up. When I ran a railway station, I got fed up of having to tell customers that the tickets they'd purchased weren't valid for travel. (For example, one couple turned up with tickets from The Trainline that were issued using an offer only valid on the trains of Company A but they were for a train operated by Company B. Further, they'd been given seat reservations on Company B's service, despite the fact that no reservations were available on that service, as the seats weren't even numbered).
AVOID! AVOID! AVOID!