Cache is really fast memory. The way it works is, when the hard drive is accessing files on the disk, it takes those files and put them on the cache so the CPU can access them. Cause some times the Hard drive will be able to access data fast, while other times, when the data is fragmented, it will be slower. So the cache acts as a buffer between the disk and the CPU.
Now if you have more cache, then there will be more amount of buffer available for the hard drive to dump data into resulting in faster data processing and such. Now this does not 'necessarily' transform into better performance, cause file systems are quite efficient nowadays and disks are also pretty fast. In most cases it does, but it also depends on other factors. But you can't go wrong with more cache....
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