Why do scientist use estimations?

Okay, I'm an English teacher. The only estimations I use is how long it might take me to finish a book. Still, I think that I have this one! There are certain phenomena that Scientists can't get an exact number or conclusion on so they estimate within acceptable parameters. Consider the age of the Earth. It would be impossible to get an exact age so Scientist estimates. The same goes for dinosaurs, pre-historic man and how many times Oprah has lost and gained weight (okay, just joking on the last one!) Also, scientists estimate logical conclusions when they don't have the time or money to conduct a given experiment. I hope that answer was okay, I "estimate" it might be!