Soil contains macronutrients and micronutrients necessary for tree and shrub growth. Characteristics of the soil and level of these nutrients available varies from site to site. That's where soil testing comes in.
A soil test analyzes nutrient levels, pH level, organic matter and nutrient retention capacity in a particular soil. The purpose is to identify deficient nutrients as well as any problems with pH level or amount of organic matter present.
Testing reduces the potential for applying unnecessary nutrients and assures fertilization adequately addresses problems. That means fertilizer and money spent on that fertilizer aren't wasted by "guessing" and trying to correct a problem that doesn't exist.
Results of a soil test should be used in conjunction with horticultural knowledge to make decisions on how to improve plant health. That may mean the need to add nutrients through fertilization, to amend the soil with compost and biochar, to adjust the pH level or all of these. Properly adjusting soil conditions with test results will yield improved appearance and growth.