Structured exercise is by far the most important piece of advice we give to our clients. Why, because 90% of unwanted behaviors are due to the fact that your dog is bored and not getting the physical and mental stimulation they need. If you have a pup that jumps up on people often or behaves frantically when excited, then regular exercise, usually early in the day, will typically make a difference.
Exercise can help with anxiety and aggression. A study in Finland found that the daily amount of exercise was the largest factor affecting whether or not a dog had separation anxiety or noise sensitivity. (Early Life Experiences and Exercise Associate With Canine Anxieties https://pubmed.ncbi.nlm.nih.gov/26528555/ ) Also, dogs that take shorter walks are more likely to be fearful than those that take longer walks. Another survey from Bristol University in the United Kingdom found that the more that people play with their dogs, the fewer behavioral issues (such as whining, jumping up, and not coming when called) their pets have. (Dogs: Their Secret Lives https://www.markevans.co.uk/television/dogs-secret-lives/
From a training perspective, exercise is so critical. Dogs were selectively bred over thousands of years for one common purpose: to work with, cooperate with, and take direction from people. A common trait selected for many dogs was energy-- and lots of it! The more stamina and endurance dogs had for those long work days, the more valuable they were to us. Something has changed though. We no longer require dogs assistance in most of our work days. Many of us go to work during the day, and our dogs are left home with little to do. Dog's genetics haven't changed that much, so many of them still have a natural desire to work with and play with people.