Given that compute is often priced proportional to (maximum) memory usage, there is potentially a lot to be gained: dramatically cheaper hosting costs. Of course if your hosting costs are small to be begin with then this likely isn't worthwhile.

Well, that depends on information you haven't provided. Maybe your system does have an extra 900 MB of memory hanging around; I've certainly seem systems where the minimum provisionable memory[1] is more than what the system will use for program memory + a full cache of the disk. If that's the case, then yeah, there's nothing to gain. In most systems though, 900 MB of free memory could go towards caching more things from disk, or larger network buffers, or something more than absolutely nothing.

Even with all that, lowering your memory budget might mean more of your working memory fits in L1/L2/L3 cache, which could be a gain, although probably pretty small, since garbage isn't usually accessed. Absolutely nothing is a pretty low barrier though, so I'm sure we could measure something. Probably not worth the engineering cost though.

There are also environments where you can get rather cheap freeing by setting up your garbage to be easily collected. PHP does a per-request garbage collection by (more or less) resetting to the pre-request state after the request is finished; this avoids accumulating garbage across requests, without spending a lot of effort on analysis. An Erlang system that spawns short lived BEAM processes to handle requests can drop the process heap in one fell swoop when the process dies; if you configure the initial heap size so no GCs are triggered during the lifetime of the process, there's very little processing overhead. If something like that fits your environment and model, it can keep your memory usage lower without a lot of cost.

[1] Clouds usually have a minimum memory per vCPU; if you need a lot of CPUs and not a lot of memory, too bad. I don't think you can buy DDR4 SIMMs of less than 4GB, or DDR5 of less than 8GB. Etc