Hmm that's an interesting one. Often you would have some kind of grease trap on a kitchen sink outlet like that. But I can't see how this unit would work as a grease trap, from the pics shown. Also, the kitchen drain pipe coming in the top would not be kosher.
Is that round black top lid that covers the hole an open grating that lets rainwater from the yard drain down into the drain? It looks kind of like it but I cant see for sure. In that case, the funnel is probably to restrict the flow of rainwater coming in so it does not enter faster than the drain can take it away. Otherwise the rain water might back up the other drain pipes.
I don't know about the plumbing rules where you live but most places you don't have kitchen grey water from the sink going into the rain/stormwater runoff system. Kitchen usually goes into the sewerage system along with toilet and shower water etc and rain stormwater runnoff goes into separate drains. So maybe something is not quite right there? Different places have different rules on that though, so maybe not.
EDIT: I think the above post nailed it while I was typing.
I wouldnt leave the funnel out though. It must be there for a reason. And it pays to toss a bit of anaerobic biological drain cleaner down the sink once a week or so to keep that fat from forming "fatbergs" again.
Edited By Hopper on 21/02/2022 11:11:11