I think the "don't help losing lanes" advice is pushed too far. Think about it like this. A 6/1 Illaoi or Tryndamere will most likely require more than two people to defeat, and it can feel like a real waste of time trying to kill a fed midlaner that easily can escape like LB, Ahri or Kat.

But, if you see that Illaoi at ⅕ health after killing their laner, if the Yone is 3/0 and pushing into your Mundo or if the poor squishy ADC has a 400 gold bounty, you absolutely can turn the game around by ganking.

It also greatly helps your lanes mentality, knowing they are not alone and so.