The "lifetime" UA Miles represent the number of Miles you have flown on United or Untied Express, only. If you fly on another Star Alliance ( BD, LH, SQ, etc) or through a credit card, they will count toward your redeemable miles and elite status, but not your "lifetime" miles. They are ONLY UA and UA exp flights.
Some airline (forgot which, maybe Delta) used to have a million miler status.
It is either another elite status or you get some one time perks once reaching one million lifetime miles... again can't remember clearly.
So there could still be some use for tracking this.
On Delta, you have that million miler thing. 1M you get lifetime Silver status plus a gift: 'an exclusive gift of elegant Hartmann luggage'. 2M Gold, and 4M you get Platinium. I am about 80% of the way to Silver, which is quite nice... now I can start using another airline, and not worry that much about reaching my status qualification every year. Silver benefits are good enough for me.