To the original question first, I feel there are several benefits to a cover, be it glass, hood or Plexiglas. Restricting water evaporation into the room (which can affect your walls and ceiling) and retaining water in the aquarium, maintaining a moist and warm air space above the water (some fish like anabantids, corys, etc will appreciate this and anabantids need it), keeping dust out of the tank (wiping off the top of the glass and light fixture regularly will show you just how much dust would have got in without the cover), and of course keeping fish in where they belong. Some fish will always jump out if not covered, but many fish that would not normally be considered likely to jump will from time to time, especially at night if startled by something.
To Izzy's question, not likely. After all, if raising the tank light fixture a couple of inches above the tank, directly, can impact the light entering the tank, you would need a lot of very bright light in the room before it would have any impact. I have the standard tank hoods with the light fixture over my 10g, 20g, 29g and 33g tanks because they came with them. My three larger tanks have glass canopy sets and then a dual-tube fluorescent fixture across the centre.