Why some two stroke petrol engines need to mix oil in it?

Why some of two stroke petrol engines need to mix oil in it. and some of won't. i drive 2 stroke Yamaha RX 135 with petrol as fuel. but it aint require oil to mix within it.
I know the whole orientation of 2 stroke and 4 stroke is different as there is no sperate provision of oil sump in 2 stroke as that of 4 stroke engines.
I guess its good purified petrol itself is self lubricated that it will carry out its performance at desired level .so no need of extra adding of oil is necessary in 2 stroke engines.
But its not same for all bikes and scooters(old ones).
Well modern bikes and scooters r 4 stroke now so this question is only for old bikes lover.

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