Well 1st off a lot of people confuse BRUISERS for tanks so you might think a team is tank heavy even if they actually only have 1 tank.
but even if a team does have 2 tanks it could be needed.
You know how many times i've been in a game as Leona and had people on the team complaining about having no peel? Well maybe if our top didnt lock in Yasuo or our jungle didnt lock in a ADC then maybe someone else can ENGAGE the fight and let me peel or I engage and they peel.
When you pick a squish comp you just put more targets on your team for the enemy to focus. If you engage then you have no abilities up to peel and if you save your abilities to peel then who is going to engage?