Letters can be attributed to a number, algorithim, process, unknown variable or anything you want to equal it to. For example if you would like to shorten the length and manipulate something more easy. This applies in calculus lots.
For example though you might have 1,000,232 as a number and like using it a lot in one calculation, thus if you let it equal a letter such as v, you can now simplify the amount of work you have to do. You might think this is not a big number but the work to write it depnding on how much you have to could attibute it as long, or when you have to do small adjustments it's easier to maybe have v-10 for example. Maybe you are working with manipulating google plex or other large numerical quantities. So by the end of a long process you might have 20 v, wich was only v written 20 times, and you can now add the 1,000,232 (20 times). As well, an example is the use of pi, theta and other symbols that have puposes other than for numerical values.
