First, it's a round number with three 0s. The next round number will be 2000. These are only two round numbers for every chess player. It's also the first 4-digit number a child can achieve.
Second, crossing over to 1000 often means you are no longer a beginner. You may be labelled as an advanced beginner, but you are not a novice beginner anymore. You already know a lot of tactics. You can play some pretty decent games. Often, a student can easily win school championship when s/he is above 1000. S/he could perform well in the lowest section of any major adult tournaments.
Third, from the Step Method point of view, you are more likely having passed Step 3. Your knowledge already includes most tactics, some openings and some endgames. You have played maybe 10-20 tournaments and performed well. It's time for you to further your chess study.
Fourth, most students are becoming more serious about chess after they cross over 1000. They will be more confident about what they can achieve. Sometimes they can zoom up very quickly. Sometimes it takes a few years for them to become an advanced player. But this is the demarcation line that they start working hard on chess.
Therefore, we always celebrate this moment.