Graphics professionals can easily chew up that RAM. After Effects, for example, a common use of Pro Macs. After loading the OS, After Effects, and typically several other apps into memory, let's say you have 24 MB available for data (from 32). You also want a little headroom, you definitely don't want to be bumping up against the RAM limit, especially while rendering, as that can drastically increase render times.
If you're processing uncompressed 4K at 32 bits per channel, that's about 133 MB per frame. That only gives you 180 frames of preview cache. At 60 fps, that's 3 seconds, which...sucks. That's pretty much useless. And that's not even including pre-comp cacheing, which you will typically have quite a lot of, using even more RAM.
Now, let's take a less extreme case, by today's standards. Let's say you're processing 2K (1920x1080) at 30 frames per second (or maybe 4K, previewing at only 1/2 res), at only 8 bit per channel color. This is about the lowest-end specs you'd expect to work with today. Now you're only at about 8.3 MB per frame, which lets you cache about 2,900 frames. At 30 fps, that's still only less than 100 seconds of preview. Still not great and again, not including cacheing of any pre-comps.