For RNA transfection, serum is avoided as it may contain RNases, which drastically reduce the amount of RNA reaching the cells. And also, if the transfection is based on a cationic lipid method (lipofection), some of the serum proteins interfere with the lipid-RNA complexes, which also reduce the efficiency (this reason also applicable for DNA transfection). For some cells, the absence of serum is also important to bring the cells to a G1/G0 synchronization, which is the best phase of cells for transfection and gene expression. Some cells also need serum depriviation just to increase the pinocytosis which engulf the transfection material and increase the efficiency. I hope I could answer your question. Good luck