Training on your own can have many benefits, you can do exactly what you plan without someone else either slowing you down or making you go to fast, to long or convincing you to do something different or even finishing early.
Training with others is good as it stops you from missing a session and can be very social with other like-minded people. An easy session is always good to do with someone who is slower than you, to help you keep the pace low, however the other person will be forced to try much harder.
Training on your own is hard without others to keep you going. You can tend to change pace without noticing, this is far more visual when with others.
Many athletes find it difficult to train on their own; some of your training should be done on your own. When you compete unless you are in a team you are really on your own so solo training helps. A long training session can be much mentally harder and feel longer on your own. As a result you can become mentally tougher from solo training, when with others the time does seem to go much quicker.
It is easy to slow down when you are tired towards the end of lonely training session without the motivation of a training group.
No one is there to notice any differences in style that can lead to injury.
Signs of over training.
If you feel too tired to train, ill or believe an injury is approaching you can much easier decide not to train, or not complete the session planned, if you are on your own.
Solo training allows you to concentrate on breathing style economy, as others do not distract you.
Much easier to keep within pre planned heart rates when solo training, without others dictating the pace.