Why do we use pressure instead of force for liquids and related measure? I have been reading about pressure and wasn't able to find the actual meaning of pressure except for the formula pressure = force/area. And when I began to study Boyle's law, need to understand the pressure grew. So please tell me why do we use pressure in case of liquids?
 A: Pressure is a scalar quantity, while Force is a vector quantity. It is quite meaningless to speak about force in a liquid at equilibrium because there is no "direction", the liquid is made of particles randomly moving. Therefore we talk about Pressure, which more or less signifies how hard a liquid "pushes back" when compressed. 
A: So please tell me why do we use pressure in case of liquids?
We use pressure in the case of liquids and gases because the force they exert on objects is distributed over the area of the object. There is no single “force” being applied.  
The pressure of a liquid or gas is the force the gas or fluid exerts perpendicular to the surface of an object per unit area over which the force is distributed. If the pressure $p$ is uniform over the area $A$ then
$$p=\frac{F}{A}$$
or, the total force is 
$$F=pA$$
Where $F$ is the magnitude of the force normal to the area of contact.
Force $F$ is a vector, pressure $p$ is a scalar, and area $A$ a vector perpendicular (normal) to the area with magnitude $A$. 
If the force is not uniformly distributed, then can take a small differential amount of area and look at its contribution to the total force due to pressure. Once again the differential element of area, $dA$, can be considered a vector perpendicular to the element of area and having magnitude $dA$. Then
$$dF=pdA$$
(Vector=Scalar x Vector)
or 
$$F=\int pdA$$
Hope this helps.
