We all have moments where it's hard to pull ourselves out of bed at 5:30am to head to the gym.

 Training with a group can often give you the extra motivation you need. Having like-minded people training alongside makes you feel like you are not in it alone, and the banter is always good!
Alongside this, the support you get from your trainer and the other people alongside you feels great.
The classes are usually well structured, meaning you never really have to think about what training you have to do, and no planning is ever needed. 
The only thing you have to do is turn up and train!