Those are considered your total BIS miles (butt-in-seat), the lifetime total for actually flying. Actually, they are your lifetime total MQM's/EQM's. Therefore it includes bonus MQM's from YBM and F fares, from AMEX bonuses, etc. There are some benefits and recognition for hitting milestones; in particular 1MM, 2MM and 4MM). Million Miler miles simply serve for keeping a running tally as you attain those milestones.

Some other airlines calculate lifetime miles towards Million Miler status in very different ways; for example some include total RDM's (redeemable miles) earned. In the past IIRC, DL did that too, so it was easier to become a DL million miler 15 years ago than it is now.

I believe the top current customer is somewhere between 7 and 11 MM.