Indeed, mass-spamming silos is mostly a late game thing to bank a bunch of stuff to defeat a big crisis.Sure, at some point you're going to have access to so many planets and so many resources you don't know what to do with them. But by then you've won, and silos didn't make that possible.
That said, it can be worth building at least a few silos before that for two reasons:
- Dealing with massive resource spikes. If you federate with a megacorp and run Golden Rule, they'll happily "donate" their entire treasury to you every 10 years. In my most recent game the election pot totaled over 100k energy credits in 2260.
- Overall resource fluctuation from conquest. Buying/selling stuff on the market is *brutally* inefficient after the early game, to the point where you never really want to use it. Conquest can often lead to you owning weirdly suboptimal worlds that can cause your income to spike in some resources and plummet in others. Evening things out takes years.
Those 0.1's add up.
If you're generating say 10k science/month (most likely you're doing quite a bit more), that's about .5 pops you need to compensate with per extra depot.
A few of them could be good, sure but across an empire, if you have breeder worlds, those worlds likely have spare building slots anyways. You can fit them there, but once you're leaving additional city districts in place to get building slots, now you're paying for both the district and the building in your sprawl. At 20k science/month it's now 1 pop per, or 2 pops per if you're building a district and building to get the depot.
Essentially it's like free population to not build more of them than you actually need.
Two things here:
- I was actually incorrect when I said it was 0.1. You actually only gain 0.5 sprawl per district, not 1 like I had thought. So it's actually 5% tech cost per million storage, when you're probably already getting +500% from pops/colonies/etc.
- Buildings don't directly contribute to sprawl at all. I'm not sure if you're referring to a knock-on effect of needing to build other districts to pay for maintenance or something, but buildings are sprawl-free themselves.