It helps in 3 main ways:
Consistency – Not missing sessions because somebody is waiting for you.
Work Level – You always work harder when somebody is next to you doing the same thing, or shouting at you to do one more rep!
Quality – Having somebody watch what you are doing, even if they are doing it themselves means they can sometimes see any incorrect technique.
I advocate training partners but not all the time.
At least some of your training should be alone.
Squash is a solitary sport. We have our opponent to play against and that drives us to greater heights but when it comes down to it we are alone on court.
Alone, but with the training behind us.
However, if we only ever train with somebody, when we are about to give up during a match we become accustomed to expecting a “push from our partner” but it won’t come.
Yes, we might hear them from the balcony but like boxers we are alone in the ring/court.
Training alone is tough but it make us mentally stronger.
That slight reduction in work load compared to training with a partner is worth it in my eyes for the extra grit you develop.