Show up and strengthen someone

Heiser is honest about this verse. In the church context he grew up in, it "was used basically to justify" turning up whenever the doors were open - and if you did not, something must be wrong with your walk with God. He names that as an abuse of the text, while still holding on to the text.
Because look what it actually follows. The writer has just finished arguing that the sacrifice is complete and needs no repetition. That is all you need. Therefore - gather. Not to top anything up. Not to prove anything. The purpose is given in verse 24, and it points away from you: consider how to stir up one another.
Scripture links:
- Hebrews 10:24-25 - "let us consider how to stir up one another to love and good works, not neglecting to meet together... but encouraging one another, and all the more as you see the Day drawing near."
- Hebrews 10:18 - Where there is forgiveness, there is no longer any offering for sin. The gathering rests on a finished work.
Go for what you can give someone. Attendance was never the point.