Because cells need amino acids to produce proteins. Proteins are just long chains of various amino acids connected together by peptide bonds. They are probably the most important substrate in the body, and do various different jobs in the cell, like allowing larger substrates to diffuse into cells, and acting as hormone receptors in cells. Amino acids travel through the blood as products of digestion, where they diffuse into the cells where they are needed.
Well you that the concentration gradient has certain mechanisms which are that fluids move from higher concentration to a region of lower concentration.
Because they need to make proteins
The cell needs the amino acids to make protein
Because the body needs them in the bloodstream and it is essential.