<p>Do we need mold release agent in 3D printing mold? If it is not used, what effect will it have on the product?</p>

Welcome to the 3D Printing Stack Exchange site.
Used in Casting
A mold release agent is commonly used when a part is cast.  The release agent is placed on the inside of the mold before the liquid object is added.  As the object becomes solid, the release agent prevents the object from adhering to the mold.  As a result, the objects are easier to pop out of the mold, and in some processes, the mold can be reused.
3D Printing is Different
A mold release agent is used to allow the desired part to be separated from the mold.  In FDM (thin plastic extrusions bonding together into objects) 3D printing, the object is surrounded by air, except for the bottom where the object contacts the print bed.
Bed Adhesion
For most materials, getting the bottom of the object to stick firmly enough is the problem faced, rather than making it easy to remove.  In many cases, a compound is placed on the top of the bed to help the plastic stick to the bed.  It is a "mode adhesion agent" rather than a release agent.
For some combinations of materials, the bed material and the plastic have a particularly strong adhesion, such that it can be difficult to remove the object without damaging the bed surface.  Notably, this occurs with a PEI bed and PETG plastic.  In this and similar cases, the mold adhesion agents can be used on the bed.  This slightly separates the plastic from the bed material, and we can avoid bed damage.
Internal Adhesion
With multimaterial printers becoming more common, there are cases where two parts which might touch and stich during printing should be isolated during the printing process.  A second (or third) material can be used to isolate the parts.  If the isolation material is sufficiently different from the desired objects, it can be removed by a solvent.
This approach is limited to cases where the objects should be separated by at least one printer thickness of the soluble material.