Why is your 1RM important?
Your One Rep Max is the absolute maximum weight you can lift for a single repetition. Knowing your 1RM allows you to program your workouts using percentages (e.g., training at 70% of your 1RM for hypertrophy, or 90% for strength).
Why use a calculator instead of testing it?
Testing your true 1RM in the gym is extremely fatiguing and carries a high risk of injury if your form breaks down. Using the Epley formula allows you to safely estimate your 1RM based on a weight you can lift for 3-10 reps.