More memory means you can cache bigger frames for longer amounts of time and enhance video playback. Why memory is so important: Memory enables real-time video playback in After Effects, and it powers the RAM Preview and Global RAM Cache features that hold and recall frames and compositions from projects you’ve recently previewed. You can install even more RAM for longer previews and Global RAM Cache memory, and to run other programs at the same time as After Effects.” 1
Although the stated minimum is 4 GB of memory, if you plan on taking advantage of multiprocessing, you should consider installing as much as 48 GB for a 12-core system and 64 GB for a 16-core system (check with the manufacturer for optimal memory configurations to complement your CPU).
What Adobe says about the role of memory in After Effects: “If you plan to primarily run After Effects CC, then you will directly benefit from installing more RAM. Everyday actions that rely on memory: Creating motion graphics, applying effects, previewing files, real-time video playback, utilizing the RAM Preview and Global RAM Cache memory features, and running After Effects and other applications.
