Trainers can be pretty pointless, they usually bank on people not knowing anything about how to get fit and not caring to know, which is why they're paying a trainer to just tell them what to do. A trainer will often be more focused on making workouts either fun or exhausting, so you'll want to come back and/or feel like it's working (and while being exhausted usually means you've burned calories, it doesn't automatically mean you're moving towards a specific goal like strength gains etc). But by doing this, their workouts are often structured in a way that takes you through a million different exercises, leaving you with no strong understanding of any one of them, and no knowledge to really take away from it to use in your own time.
For people who just want to get their heart rate up for an hour 3 times a week and don't want to think about it, a trainer can be useful to keep them motivated, but if you're training towards a specific goal, you'll want to build a foundation of knowledge for yourself beforehand, so you can either guide yourself or select an appropriate trainer/coach who you can trust knows what they're talking about. A good lifting coach, unlike a generic personal trainer, will be much more focused on educating you.