Funnel type water filter-Is it necessary?

Hello,

We've drilled a new well and are installing tanks and filters. Everyone seems to have (and the shops say I need one) a filter type device that sits on top of the first holding tank that the incoming water from the well goes through. I'm sure you all have seen them. They are like a square 3-tier funnel looking thing. The top two levels have multiple holes in them to allow the water to filter downward. The bottom one just seems to be a funnel to direct the water into a smaller tube allowing it to drop into the tank.

I was told this is for oxygenation and would help the solids settle to the bottom of the tank. It looks to me as if it's only for filtering out big rocks. My question is two fold. One, what is the actual purpose of this device and two, how necessary is it. I don't mind spending the 2500 baht if it has a good purpose. If it's just because "that's how we've always done it", then I don't want to waste the money.

Thanks for any suggestions.