FG did a better job breaking this contract's value down than I did:
https://blogs.fangraphs.com/rookie-standout-michael-harris-ii-signs-72-million-extension/
"Under the old CBA’s service time rules, ZiPS estimates the Braves would have had to shell out approximately $41 million over Harris’ six cost-controlled seasons and then $57.5 million for two years of free agency, totaling $98 million over eight years. But Harris’ rookie campaign changes those calculations, as under the new CBA, a player who finishes in the top two in Rookie of the Year voting gets a full year of service time. My award estimator gives Harris an 85% chance of finishing in the top two in the NL Rookie of the Year race; hitting free agency a year earlier boosts the system’s estimate to $123.5 million. Sum it up, and you get a valuation of about $120 million over eight years."
So the Braves got $120M of player value for $72M. Pretty good value.
However, these early deals are not without risk. This article only touches on it:
"ZiPS doesn’t expect Harris to maintain quite this level of performance, and it isn’t enamored enough with his approach at the plate to give him a huge peak."
Harris does have a high chase rate, and it's hurting his BB rate. If he doesn't improve upon it he will be stuck being a power vs hit version of Inciarte. Still valuable, but a bottom half hitter. This is doubly so if he doesn't improve vs LHP and needs a platoon partner. At that points he's a 2+ win guy signed for 8 years.
The good news is that he's 21, and projection systems like ZiPs are not good at projecting young kids truly improving their skills. For example, ZiPs had Harris as a .242/.294/.380 line entering the season because it has no idea how to predict someone like Harris improving very real skills in very real ways very quickly. So it wouldn't shock anyone if Harris figured out some plate discipline, figured out how to handle LHP a bit better, and became a 4+ win CFer.