"There is widespread consensus" as they say, that computer memory is a little confusing. They'd be right.
Back in the bad old days this problem didn't exist. You had a box the size of a fridge that was your core memory array, the magic part of the computer that was its memory and cost more than every house in your subdivision put together. That was memory. It became suitcase sized a deacde later and now it's something you buy for $9 as an attenion seeking item staring at you from the hey-see-me checkout display at the hardward store, in the form of say, thumb drives.
This has impacted how computers are built today and they now have a few types of memory. Lets get disk out of the way. There are two types of desktop and laptop memory devices in the computer: disk and RAM. This is not the strict definition, but work with me here, ok?
Disk memory stays in place when you turn the power off. Right? Say it again Disk memory stays in place when you turn the power off..
That's the big difference. RAM on the other hand goes away withing seconds after you turn the power off.
Disk memory is where you store your files - text, pictures, music - so they're still there when you turn the power back on.
So lets forget about that. What we want to look at is the memory that is a bunch of chips we have to plug in to slots inside our computers whether on the mainboard or some card we plug in like a video card or RAID controller - computer memory, usually in the form of "sticks" which are just little circuit boards with a few RAM chips on them.
Your computer can "think" because it has a memory, and it has a CPU that can manipulate that memory. Back in Old Skool any program gave the appearance of moving images by drawing a picture, showing it to the user on a display, then doing it again in less then a twentieth of a second. With the limits on the CPU you usually didn't get much if a picture - even at a couple of gighertz there isn't time to, say, redraw the shaded texture lit up by the sun that makes the game look so cool.
So, a CPU was put on the (better) video cards so they could do a lot of the "grunt work" enabling a more complex image to be generated in that precious twenty-fourth of a second.
You already know that cpus can do more stuff with more memory so it always makes sense to get as much as you can. The video memory is used by the CPU on the vido card. The memory on your motherboard is used by the CPU on your motherboard. Ideally you want each to have as much as they can get, that is as much as that hardware allows to be plugged in.
It's the same with RAID controllers - you can get cards with caches varying up to 16X the size of the cache. It's a disk accelerator card and the more memory you throw at it the better it accelerates.
Brian Reid once said something like: "How's how to tell if something needs more memory. Does is have a power cord or a battery? If it does it needs more memory." and he's quite right about that.