1. Pretty much anything written in java or jvm languages (clojure, scala) needs a lot of ram.
2. Since (at good hosts) there are fewer high-ram vps to a node than with low ram, you tend to get more cpu and i/o performance even if you don't use the ram.
Still though I've never made much use of high-ram vps's (512mb or more). The extra ram comes in handy for occasional spikes like compiles. But for ram-intensive processes I've always used dedicated servers so far. I ran a search engine for a while (few hundred GB of data, tiny compared to web search) that served queries without too high cpu load, but rebuliding the index took 100% cpu for several days. It's not really feasible to do that with vps's.