Hello, because of with the use of water as mobile phase you will have final purified product in water which by evaporation you will have pure polysaccharide without any salts and ions which can come from buffer used as mobile phase.

you can use a buffer as long as it doesn't change the ionic form of your anion exchange resin but indeed this will result in a salt rich polysaccharide fraction.

Buffers or neutral salts are used for elution (binding in water or very low concentration buffer if pH needs to be controlled).