Hello and Welcome to TFP!! Exciting times to have your first pool installed.
The main drain allows water to be pulled from the bottom of the pool and not just through the skimmer. Many feel that main drains are not necessary to have a clean sanitary pool. Once you have multiple ways for water to exit the pool into the pump, then you need to consider if they are balanced.
If you decide on a main drain, they must be anti-entrapment compliant. You should also have the main drain hard piped out to a point where you can add a 3 way valve to control the flow from drain vs skimmer.
A main drain is another place for a leak to occur.
The drain does offer some benefit to be able to control circulation better. AGPs are notoriously poor with circulation. Almost always, the return is within 2 feet of the skimmer.
Is $300 worth it, maybe. It is probably $100 in parts.
How are you chlorinating the pool? Do not install any UV system, no ozone, nothing that will put copper or metals in the water. This can and will stain the pool surface if allowed to buildup in the water. No single speed pumps. No cheapo slide valves, or other cheap valves. Make sure there is at valve at the inlet to the pump and outlet of the filter at a minimum.
Ask any other questions you have.