In every field or discipline, a lot of tasks are repeated over and over.
Think of tasks you usually do —they don’t necessary need to be work tasks. To give a few examples of activities I do often, I can think of formatting a document for print, doing a photomontage, rendering a 3D scene, drawing architecture sections or plans or writing some code.
For each of those activities, I frequently need a set of resources in order to complete the job. This resources can be fonts, textures for renders or photomontages, fragments of code. Anything that can make your tasks easier in the future.
From time to time, assets you have used before appear to suit the needs for a current task.
Creating your own personal library will increase your efficiency in tasks that you often do. Your library should be customized to your needs and divided into flexible categories of resources that you frequently use. It should be organized and up to date. If you don’t do so, you will find yourself searching or creating things that you already have in your library, but can’t manage to find.
This is how my library looks like. It is also subdivided in smaller categories. Again, it is important to have a flexible library that can implement new categories that you do in the future, or remove the ones that you don't do anymore.
Templates serve as a base file when creating new documents.
After formatting documents with Adobe In Design, I created a template file, with the font size, typeface and paragraph style I use. When I need to format a new document, I use my template as a base.
In AutoCAD, I have a template file too. When a new file is created, it already contains my layers, line widths, typefaces, printing formats and other custom settings.
Assets are stored elements that you can use in future tasks.
I frequently go back to code snippets I have done before and reuse or modify them. When doing 2D and 3D graphics, I have material in my own library, such as vectors, textures or photomontage items, so I don't have to search or create new ones each time I need them.
Using templates and reusing assets are the main situations I can think of that will make your library valuable. This are just two examples of tasks I do, and the important part is that you can save time in the future when you need perform a similar task.