Combo start. Combo damage is influenced by attack and ninjutsu depending on the type of chase.

Take Mei for example, high combo stat will make her more likely to land her "special" normal attack which causes acupuncture and knockdown

It's only useful for characters whose attack have such RNG "specials". Sage naruto, for example, does not care for it as he ALWAYS trigger low float (or summon a clone, if there is none) so it have no offensive benefit for him. Then there are boring chars whose normal attack never cause anything, like Baki, also no offensive benefit there.

That said, combo stat is a comparative stat between the attacker and defender, so most chars would benefit from having it defensively, that is to say the enemy is less likely to combo them. So it is always somewhat useful.

As far as 8 gate goes, only character with REALLY special normal attack(causes a chaseable status AND a debuff), such as mei, should be carrying one. It's generally not worthwhile for other character to spend a slot on this.