Right now, I am in college for climatology, but before I decided on climatology, I was taking classes in Meteorology, the study of weather. So, I will answer your question. All meteorologists like to be right, but since the weather is so spontaneous, it is really hard to be right. Meteorologists use percentages because it helps them be professional. Also, because, different parts of the region where they are may or may not get the rain. I don't know where you are from, but I will use my hometown. In McAlester, Oklahoma, there may be a 70% chance of rain, but in another part of the region at Tulsa, Oklahoma ( which is 91 miles north of McAlester) there may be a 40% chance of rain. The difference in the percentages can vary from how stable the environment is, altitude, location, and terrain. That is two of the reasons why Meteorologists use percentages to predict weather.
Giraffes and Elephants: Giraffes have adapted a long neck over time letting them eat and access food allowing them to survive. If a dry season was unusually long they would succeed fine because they are able to eat and drink from the trees. Elephants have survived with similar tactics of eating higher up, they have rough skin allowing them to outlast the heat, and only need water after long periods of time.