Can scientists use global temperature data as is?
No. To understand why not, imagine you're a nurse checking a patient's chart. You find the following temperature readings (Fahrenheit) for the last few hours: 99.2, 99.8, 1000, 101.4. You'd know immediately that the third number was a mistake. To make a realistic assessment of the patient's condition, you'd have to either adjust it or throw it out.
Weather observers are as human as nurses, and they also make occasional mistakes in recording and transcribing their observations: impossibly high highs and low lows, the exact same temperatures for two months in a row, etc. The first step in data processing is quality control: identifying and eliminating erroneous data.
For more information, visit: https://climate.nasa.gov/blog/3071/the-raw-truth-on-global-temperature-records/.