The baffles allow air to flow up on the underside of the roof decking keeping it cool. In winter that's important because it helps prevent ice dams. They cost a couple of bucks each and shouldn't add much to the price of the job.

You should consider an unvented attic structure if this is being converted to living space. It takes a lot of physical depth of fiberglass insulation to get the proper R value required by code for a occupied space, so I hope you have enough ceiling height to have a legal headroom as well as the code required insulation level if this is to be occupied space. Spray foam will give the most R value per inch so it's easier to get the needed insulation values with the least amount of loss of headroom by using it. You need to consult your local building codes office for guidelines as to what is required here. Or look at the IRC codes if you live in an unincorporated area. Those are the minimum standard to which any structure should be built. You can always built it better if you want a more comfortable or more energy efficient space.

You need to tell us the climate, the insulation used and whether or not the roof is already vented.

I suspect it is already vented at the ridge and eaves and the contractor is putting in baffles to allow that system to work when insulation is put in the rafter cavities.

Roof vents don't draw enough air to cool the roof much; their primary purpose is to allow moisture to escape.

If this is an existing structure your code may allow the rafters to be filled with fiberglass whatever the R value but you might want more insulation. However, without knowing the climate I can't offer you advice.

The weather is in Athens, GA not very cold in winter and hot in summer. I have been told the attic rafter vents can protect the roof. I wonder whether the attic rafter vents is good for the insulation, meaning keeping warm in the winter and cooling in the summer as effectively to save the energy and cut the air conditioning bills or the attic rafter vents has no much effects on the room insulation.

The eave vents, ridge vent and insulation baffles allow air to move along the bottom surface of the roof sheathing protecting it only because it allows moisture to escape. In the summer it might also reduce the temperature of the roof shingles by a degree or two but not enough to extend the life of the shingles. A lighter color would extend the life of the shingles more than venting.