Because once through thebgate, there’s the line of actually getting into your seat because some people tend to stand in the aisle and block everyone while taking their jacket off. And if you’re in a prioritized boarding group you can circumvent most of these by getting into the plane early, which results in more time watching youtube while sitting on my ass.
Source: frequent flier. In fact, I’m typing this from seat 9F on an Airbus A320neo right after sitting down.