There are many reasons to do exercise.
Hormone therapy has a tendency to lose muscle and gain fat, and exercise (combined with right foods) can help combat that.
Strenuous exercise has been shown to slow progression of PCa. There was anecdotal evidence of this for years, but more recently, trials of those on active surveillance showed that those undertaking strenuous exercise stayed on active surveillance significantly longer than those who didn't, and needed to switch to active treatment sooner.
Another big concern is loss of calcium from bones, because without testosterone, your bones will not be laying down much new calcium, but the process to dissolve away old bone calcium continues. It seems that if you are shocking or stressing your bones and your body has calcium and vitamin D3 available, you can still lay down new calcium. So taking a calcium and Vitamin D3 supplement, and undertaking exercise which stresses or shocks bones is very good. Many people here use Adcal-D3, which you can buy from the pharmacy (you'll have to ask, but no prescription needed). You might want to check with your doctor that the calcium is not contra-indicated by any other medication or condition, and your doctor might even prescribe it for you.
Finally, if you have a local prostate cancer support group, many of these run weekly gym sessions either cheaply or for free. I've just come back from one of those.