It is technically required (though I doubt you'd be penalized if you didn't have it). It's the easiest way to identify you on the bike and swim, plus a lot of it is tradition. Think back to the days before timing chips, this is how they recorded your swim split as you ran into T1. A couple of notes on body marking:
1. Put on sunscreen first, or it will smear the ink
2. Scrub it off with a washcloth in the shower (at least get a good fade), or it will imprint on your bed sheets and incur spousal wrath.
3. Most women (and some men) will immediately scrub their calf, lest they walk around for a few days with their age tattooed on them.
ECMGN Therapy Silicon Valley: Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)