Trials and appeals are where rubber meets road in the legal profession. Although most cases settle or plea bargain before either (and sometimes during), the mere prospect of trial and appeal leads to such resolution. That is because most people and businesses are risk averse, and avoid the uncertainty of leaving decisions and outcomes in the hands of trial judges, jurors, and appellate judges. Also, litigating matters through trial and appeal is time-consuming and expensive for everyone involved.
The outcome of most any trial can be appealed at least once. The rate of reversal or similar is about 25% in state cases, and just over 10% in federal cases. Fewer cases end up at the highest appellate courts, like the U.S. Supreme court at the federal level for example. Mainly this is because those appeals are discretionary, meaning the appellate judges themselves decide whether to accept the appeal. The likelihood of this is less than 10% (although once accepted, the reversal etc. rate approaches 50%).
Many appellate decisions - the written opinion disposing of the matter usually authored by one appellate judge on behalf of a panel of three or more - are published and become precedent for future cases. Transactional lawyers, regulatory lawyers and other non-litigators typically do not try or appeal cases, but they are guided by those precedents.
Though risky business, trials and appeals are crucial to the effective and ultimately efficient operation of the entire legal system.