Generally, the presence of a substantial amount of fluid (blood which is called a hematoma or serum which is called a seroma) will prevent proper healing, adherence of the overlying skin down to the deeper tissues, etc. Fluid can also more easily become infected. And its presence predisposes to scar tissue. It is best to remove it as much as possible.
But there is a flip side. Every time a doctor puts a needles through the skin and into a fluid collection, he risks introducing infection. Thus, a proper and thorough sterile preparation of the skin is necessary. And even then an infection can occur.
If the fluid is minimal in amount, the doctor may elect NOT to remove it but simply treat with a snug compression garment.
Dr Jacobs