Microorganisms are often able to adapt to new environments quickly, but they lack the specialized functions of macro-organisms. Consider that the micro-organisms of macro-organisms have developed to form machinery that builds solutions to problems, rather than being retrofitted themselves to counteract new threats. Each time a bacteria needs to adapt itself to attack, whereas the specialized cells of our body can manufacture a solution on the fly rather than waiting around for a random beneficial strait to show up in the population. It would be like a small manufacturer quickly retrofitting it's production system to produce a new product. Sure, it can produce a new product, but it takes time for setup, and the company can only produce a limited number of varied products at a time, whereas a giant production company may have a huge amount of preset equipment for manufacturing a enormous variety of products with no need for retrofitting (evolution). The larger factory could have the advantage when it comes to efficient production, with quicker turnaround.
Therefore, the specialization of micro-organisms leads to nature allowing the evolution of macro-organisms as the major factories of nature.
Excess, unused (or at least non-critical) DNA is useful because it allows evolution some wiggle room. If a bacteria suffers a deleterious mutation in a vital gene, then it's obviously not fit. But, that same bacterium with can accumulate potentially useful codes in the previously unused portion of it's genome, without needing to deal with risky mutations in vital genes. And again, as microorganisms specialize, they can fabricate new proteins and such, with lower risk, and less time, compared to micro-organisms which use evolution of their genome as the sole/primary means of short term adaption.