I wouldn't say tanks are the leaders of a party, but mechanically aggro is increased on the first person that a mob sees, so a tank "leading" the pull helps them to establish aggro. A healer being seen first can be a disaster. It really messes with threat levels on trash.

I've pulled extra mobs in dungeons like Haukke Manor when the tank is avoiding mobs outright and I'm on a DPS queuing for 30 minutes for dungeon grind EXP, but I wouldn't do that to someone who is struggling or undergeared.