Recall that early computers used tape as memory — paper tape at first and later, magnetic tape. Tape-based memory is serial: If you want a byte of memory in the middle of the tape, you have to start at one end of the tape and spool through to the location you want. That’s clumsy and slow.
In early PCs, which used cassette audio tapes for memory, it was not uncommon for tens of seconds to pass before the PC could even begin to retrieve a particular piece of information from a tape. You’d sit there twiddling your thumbs while the tape drive whined its way to the location you needed.
RAM lets you access any memory address directly, in one step, without having to pass through any other addresses first. That means you can access memory locations in any random order you wish — not just by slogging through in 1-2-3 serial fashion.
Plus, although this has nothing to do with the random-access part, RAM is electronic, operating at nanosecond speeds instead of the vastly slower speeds of mechanical tape transport. Combined, these two advantages — random access and electronic speed — make RAM multiple orders of magnitude faster than mechanical, serial memory. There’s no comparison at all!
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