USCF rating is calculated based on over-the-board games with long time control. They are the real test of a person's strength. People tend to play blitz online because we as human don't have the patience to stare at an unchanging screen for even 5 minutes, especially when we wait for our opponents (who are invisible) to move. So online rating basically is blitz rating. Blitz rating can fluctuate a lot and is not accurate. Online ratings can have huge discrepancy from USCF rating, higher or lower either way. 500-point difference is not uncommon. My chess.com rating is 400 lower than my USCF rating because I get timed out in 60% of my online games.
You can use your online rating to track your progress with blitz if you play a lot. Otherwise, it's meaningless anywhere.
If a coach quotes you his online rating, check his USCF rating. If he could not facilitate his USCF rating history, there might be a problem. I knew a popular coach whose highest rated student was 1500. He had never played any rated tournaments but still had many students. Undeniable, he had done a great job to introduce kids to chess. When he finally began to play 10 years later, his rating was stuck around 1500. He actually unintentionally destroyed the future of his best students.