The Table of Contents module uses the headers to get generated. Each header gets a unique identifier when none were assigned manually. The Table of Contents module uses the text between the start and end tags as the title to show in the table of contents.
Very good!
Now, you may not want the Table of Contents itself, but you like the idea to have identifiers in all your headers. This can be useful to create in-page links (a link with an anchor, i.e. #some-name at the end of your URL.) It is generally annoying to have to enter the identifiers by hand for each header and you may make mistakes.
Another side effect from the Table of Contents module: it can be setup to assign a number to each one of your header. It starts at 1) and moves forward to 1.1), 1.2), 2), 2.1), etc. Note that you can select your own style as well.
The solution: use the Assign an ID to each header filter. Once the Table of Contents is installed, that filter will be available in each one of your Input format. Select it and you will get the similar features as with the Table of Contents filter.
NOTE
This is what needs to be done whenever you want to use Table of Contents Block (unless you want include an identifier by hand, in each one of your headers, for every page...)
If you want a Table of Contents on a few pages, but not that often, and also want to have identifiers assigned to headers, set both, the minimum and maximum headers, to H12.
Now the headers will be transformed by the Table of Contents filter but the resulting table will be empty (because none of the headers will match your default selection.)
Finally, when you have a page where you want to show a Table of Contents, enter the tag with the minlevel and maxlevel parameters set to values such as 2 and 3 respectively.