The best way to explain what a custom tag is used for is by showing an example.
Let's say you are selling an e-book and you use Email Autoresponder to promote it. You need to be able to quickly change the price of your e-book because you occasionally run pricing offers that only last for one week.
Each time you change the price, it would be time-consuming to go through every message in your follow-up sequence and change every occurrence of the price.
That's where the Email Autoresponder custom tags feature can be useful.
You could set up a custom tag called {TAG_price} with its value set to '$99.00' or whatever price you were selling your e-book for.
In your follow-up messages, you insert the tag {TAG_price} wherever you want the price to appear. When Email Autoresponder sends out your follow-up messages, it will automatically insert the price that you have currently defined in the {TAG_price} tag.
Any time you want to change the price of your e-book, you simply edit the {TAG_price} tag, change its value to the new price of your e-book and save. Any follow-up messages containing the {TAG_price} tag that are sent out from that point on will contain your new price.