Why do we see some bubbles attached to a glass when filled with water?

Because there is air in the water. Just the act of pouring it into a glass traps some air. And there are microscopic fissures and ridges on the glass. Air bubbles tend to form on the ridges. If you watch water boil in a test tube you will see that bubbles only form at certain places. Chemists usually add some broken ceramic just to be sure there are plenty of such places available.
en.wikipedia.org/wiki/Boiling_chip