A computer needs a certain amount of free space on the hard drive to function. This is even more true if you have a shortage of ram available for the programs to operate within. Free space is needed for two primary reasons on every computer no matter how large or small.
Free space is needed for the obvious reason that when you want to save a file, it needs somewhere to write that file. Free space is like have extra paper in a notebook when you are taking notes in class. You may be able to find a blank space on some other sheet that has already been used, but it will not be as efficient as a new sheet. It will be harder to find when you need it and harder to add more information to.
Having free space on the hard drive allows your computer to find room for your files easily and efficiently. For small files like you create in a word processor, this is not too much of a problem. But, when you want to install new software, a lack of free space can be a serious detriment. Most software checks your free space first. If your drive is short on space, it will refuse to install its files or will only allow enough for limited function of the software.
The second use of free space is a little more complicated than just having some extra paper in a notebook. Your computer needs extra room to think sometimes. In the same way that you may write down some random thoughts for future reference so you will not forget, the computer does roughly the same thing.
If you have plenty of ram in the computer and are only running one or two applications that are not memory hogs, your computer will just hold everything in its electronic memory. This is like people remembering a short grocery list of 4 or 5 items. When this list gets to be 10 or 20 items, people usually write it down.
If you are running several programs at once, this can exceed your computer's ability to hold everything in its random access memory. Every window that you have open is a new program for the computer. Games and graphics programs tend to take more memory.
Once the memory is exceeded, your computer will write out everything about one or more programs in a special area of the hard drive so it can be retrieved when you or a program needs it later. This can hog up many many megabytes of memory at one time. If it can lay a lot of memory onto the hard drive, lots of ram will be liberated for operating purposes.
When you are swapping from window to window, if you notice your computer is hitting the hard drive a lot, it is having trouble keeping enough room on the hard drive and in memory to run efficiently. Sometimes defragmenting
