Many people aerate reservoirs used in chem aero grows. Why?

[Android]

If no beneficial bacteria is used in solution, who are we feeding with oxygen? And the roots get oxygen in abundance when they are drying up from being sprayed/flooded. How does aerating help then?
It appears to me oxygenating the res does only two things (both bad) -
1) growth of unwanted bacteria,
2) NOISE

Please someone educate a noob- I don't know what to think