I didnt realise how importnt they are untill you have to train on your own.
I've trained with the same bloke for 15 years and now he's recovering from an operation, be some time before he can spot properly never alone train.
We know each others strenghts and weaknesses, always monitoring each others form, know when to push each other and when to say "back off a bit" looks like you losing form and will worsen your injury.
There is no substitue, this bloke knows when I'm genuinely fucked and couldnt be fucked, motivation is not a choice then.
Makes a huge difference to me, but then again if youre used to training alone and motivating yourself, would not matter.