Reversibility and yield
This chapter is about the 'equilibrium reactions', where products are formed that react back with the same rate.
Such reactions we call: reversible
Don't forget that there indeed also exist irreversible reactions. Reactions that really and exclusively go to one side.
The product do not react back.
An example could be the combustion: the products water and carbon dioxyde do not react back, certainly not spontaneous, to petrol and oxygen.
You could ask what the use is of a reaction in equilibrium: if the products simply react back. What is the output, what is the yield of such a reaction?
If we want a reasonable yield, then the reaction must respond to a couple of conditions.
We can influence such an equilibrium in several ways.
So we can try to put an equilibrium as much as possible to one side, the side of the products.