Is there a lesson on the use of “le” in the following sentence where it doesn’t seem necessary? Je te le promet.

Here is the lesson:

https://french.kwiziq.com/revision/grammar/use-le-or-l-to-refer-to-previously-mentioned-complete-ideas

The gist is that you use le to refer back to a previously mentioned idea instead of a concrete noun. French is more picky about this than English, where you can often simply omit this reference. But it wouldn't be French when, mostly in everyday spoken French, it's omitted, too.

Here are a few choice examples:

Je le savais. -- I knew it! (In this case you have the "it" in the English as well.)
Je l'espère. -- I hope so. (literally: I hope it)
Tu le verras. -- You'll see.

Bonjour Judy,

Without any link to the question or lesson, it is difficult to understand where this query came from.

The closest I can imagine is, "I promise to give you it" but that would be written Je te le promets

where te would be the direct object and le would refer to "it". The verb promettre being conjugated in the first person singular. 

Te could be indirect if the meaning was intended to be "I promise to give it to you". In this construction, le would represent a direct object.

Otherwise, this may appear to be some sort of homework query.

Bonne continuation

Jim