Each platter has limited capacity. Multiple platters increases capacity by increasing the space available to store data.
A Clown Based Analogy
Here's an analogy for you: you're a circus clown, and as part of your performance you stuff a dozen clowns into your mini. Circus management wants you to fit two dozen clowns into the car, but there just isn't space.
The options here are:
- Learn to pack more clowns into the car (ie increase the platter density). This might unfortunately require the disassembly of these clowns, which is akin to disk fragmentation.
- Put the extra clowns in a second car (add a platter).
- Use a larger car (increase your platter size, which isn't practical for a hard disk).
Technical Stuff
For example, a platter in a modern disk drive might be able to hold 1TB of data. If you want to market a 4TB drive then you need four of those platters.
The only other way to increase capacity is to increase your data density. That requires research and development and is relatively expensive.
Throughput could potentially increase with additional platters, if the read/write headers can move independently. I don't know if they can or not.
How data is distributed on platters is up to the disk manufacturer and perhaps up to the operating system. Hard disks have great sequential read speed, so splitting your file across multiple platters could reduce performance.
Fragmentation results in files being on multiple parts of a disk. This means to read an entire file you have to seek between disk locations, which is slower than sequential access. This is why defragmentation utilities can increase disk performance.
From the analogy above, defragmenting a disk is like reassembling the clowns you cut into pieces to stuff them into the car. However it's less messy when done with data than clown limbs.
You should read up on basics of hard disks on Wikipedia.