This is a bit of a guess but: loan to cover the cash-flow of building the stadium, then receive (or start receiving) the sponsorship that will pay off half of it.
I recall an article about one of the top PL clubs extending their stadium or building a new one, and it said that compared to Man United they were losing out on a couple of million per match due to the difference in capacity. I don't know what the ticket prices are like in the Netherlands but, if you fill the new stadium, you could be looking at a good few hundred thousand extra per home game. That would pay off the other half in two to three seasons (depending on how many matches you have).
It's probably something along those lines. I can't work out how it could be anything else. I think it's a good move in the very long-term but you're looking at a good five or six years before you see any significant benefit I would guess, and I'm not sure I'll play that long. I could do though as I'm enjoying this save a fair bit and I can't see myself starting another game until 17 is out.
Don't really know what to make of next season. I can't see us bettering our 9th place from this season. I thought we were quite lucky to be that high and my plan of raising funds has been effectively ended by the board deciding to cut my allowance to 5% of any transfers, meaning if I sold Walid Tran (my best forward) for his £2.4mil release clause I would only see a pathetic £120k of that. I did buy him for less but it makes you question why I'm even bothering. Surely they can't keep it that low. It's damaging.