Another reason that I feel this is important is because it will allow you to have the opportunity to plan ahead financially. By knowing what you are making and when you can expect the money to come in you can ready yourself for any low work periods that you may experience in the future.
One easy way to do track your income is through the use of a simple spreadsheet. This type of spreadsheet can be created in a program such as Excel. I have found that this works best for when you just need to keep track of your monthly income and when the money should be coming in.
If you are needing to keep track of invoices and how much work is due for a certain client, a paid program may prove to be useful. These types of programs will often allow you to create the invoice in their database and keep track of whether or not it has been paid.
Deciding which method would be best really depends on the type of work you do. Not all at home workers will have the same types of money tracking needs.