The short answer is: to make things easier with filemanagers.

Let's see how things would be without the AND operator, by taking the valid tagsistant query ~/tags/t1/AND/t2/AND/t3 and rewriting it, step by step. Keep in mind that you are using a filemanager. Here we go:

1. you enter ~/tags directory
2. tagsistat lists all the tags available
3. you enter t1
4. the path is now ~/tags/t1, so tagsistant must list all the files tagged as t1 but also the whole list of available tags to permit further navigation
5. you enter t2
6. the path is now ~/tags/t1/t2/. Once again, tagsistant must list all the file tagged as both t1 and t2, together with the whole list of available tags to permit further navigation
7. ...

As you can see, the result listed ad 4. and 6. are a huge set of entries. Introducing the AND operator mitigate the problem since the steps become:

1. you enter ~/tags directory
2. tagsistat lists all the tags available
3. you enter t1
4. the path is now ~/tags/t1, so tagsistant must list all the files tagged as t1 and the AND operator alone
5. you enter the AND operator to keep building the query
6. now tagsistant list the whole set of available tags, but not the files
7. you enter t2
8. tagsistant list all the files tagged as t1 and t2 with the AND operator alone
9. ...

If you still don't understand how adding an operator can simplify your life, just think how could be scrolling a list formed by 3000 tags and 10000 files at points 4. and 6. of the first list. Better split it in two lists, isn't it?