You cant put just any fluid into a vein. If any of the constituents are dangerous then you will poison the patient. If the solution is too concentrated you will cause severe phlebitis and cause clots and pain. If the solution is too weak then the cells will swell up with water and burst.
One of the problems with administering parenteral nutrition is that you need quite concentrated solutions to give the required energy. It is therefore necessary to place a catheter in a large central vein with rapid flow so that it will be possible to administer the nutrition. Smaller veins with slower flow just develop phlebitis, get sore and clot off.