I use them when something needs salt. I use them when something needs lemons. What recipe doesn't need salt or lemons or both? And they're so nice because, if you only need a bit of lemon, you can use just a bit of lemon without the rest drying out before you remember to get back to it.
The best directions I've found (and the ones I used) are here. Although the lemons will reputedly last forever, I wouldn't make more than you can use in three months or so. I made my first batch in July and by the last lemon wedge (Wednesday, this week) they were getting a wee bit over-salty and having to be rinsed quite a bit.
This is batch number 2, started in late September and almost ready to go!