Why the need to soak lentils? How will if effect the recipe if I dont soak them? Do all lentils need overnight soaking?

soak the lentils to get the dirt out. some people say its to get the gas out. and others say its to make them more digestible. but mostly its to clean them.

Another one who has never soaked lentils. However I will add to answer number 1 and say that if it calls for red lentils, add them early, they're supposed to basically dissolve and thicken the stew anyway. But if it is more like a lentil du puy that should hold its shape, I agree, add later.

Agree, I have never soaked lentils. Odd that a recipe would tell you to do so.

Never soak lentils myself, unlike beans they cook quite fast, just rinse add to double the quantity of water and cook covered for 20 m. As sam says, cook & add them separately else they get mushy

I never soak them for any length of time. Maybe 10 mins or so rinsing, to get powder off. They should cook just fine in the stew. In fact they may cook too well and become mush.
I'd cook them separately and add to the stew in the last 30 mins.