It's so low traffic that you don't want to pay the minimum $35/mo for a PostgreSQL instance on AWS maybe. Or you're required by policy to have a single-tenant architecture, but a full always-on database server would be overkill.
Could this be used to get a time edge in trading? I'm not an expert, just thinking out loud. I remember hearing about firms laying wire in a certain way because getting a microsecond jump on changing rates could be everything for them.
I'm also no expert, but from reading around the subject a little (Flash Boys by Michael Lewis was pretty cool, also Jane Street's podcast has some fantastic information)... no. I doubt you'd be on a public cloud if low-latency trading is what you're doing.
Aren't the HFT boxes usually stock exchange colocations? Each trader gets a rack (or multiple racks depending on size) in the exchange's datacenter, every rack has the same cable length to the switch, etc.