Hi,
The name custom tags is self explanatory in the sense that people have worked on existing tags like the ones in html and added upon that default functionality. We can always refer to these custom tags if it suits your needs or you will have to build up a custom tag of your own which might take some time. Hence use of custom tags. For example consider the logic tag. this is used to carry out logical operations. Also using these custom tags a lot of Scriptlets can be avoided and make the page look neat and tidy.