A glass of water will flush the drug into the small intestine where absorption will be accelerated.
Sulfonamides are less likely to produce crystallurea.
There will be more fluid in the stomach-intestine to help dissolve the medications. Diluting the stomach acidity may protect acid labile drugs. A glass of water gets them down into the stomach where they belong. Hope this helps. I think the purpose of recommended that drugs are taken with a glass of water is mostly to avoid the risk of oesophagitis.