From an ecological sense, cycles of living beings are intertwined. Breaking cycles affects life being able to live, reproduce, and die in a manageable way. Insects, animals, and plants help feed each other and help them reproduce. Fewer insects are less of a food source for animals and can't pollinate plants as much. Reduced animal populations are less of a food source for scavenger insects and don't feed plants through manure or decay after death. Fewer plants means less food and protection of insects and animals from predators. Predators overload on newly-available prey and their populations crash. It all intertwines.
From an environmental sense, the results of a narrowed biodiversity include putting more carbon into the atmosphere. In addition to warming the planet and the ocean, the ocean acts a carbon sink, acidifying in the process. This damages existing sea life that can only live within a certain pH and temperature range without sickening or dying. Some fish are trying to migrate and are dying off, due to lack of food where they are headed.
From a medical sense, burning down forests is stupidity. There are drugs that are naturally found in their rare plants, which have been developed into medications that save lives, e.g. Paclitaxel or Taxol. Only 1% of remaining rainforests have been explored for these plants and their compounds and we're stupidly burning them up. Nature has evolved many useful remedies, but we'll never know about the ones we destroyed, and that means more sickness and death.
For people obsessed with money, that medical sense translates just as well into pharmaceutical products and the resulting loss of jobs and profits.
All of these factors are linked together, hand-in-hand.