If you’re planning on canning, you can reuse your purpose-made glass jars and screw bands, as long as they’re in good condition.
The metal snap lids, on the other hand, are made to be used once. Once the gummy, rubbery seal on the lid is heated and cooled, it can’t make the same quality of seal again. And since the quality of the seal is essential to preserving what’s inside the jar, they should not be reused. This is also one of the reasons why you should never reuse just any old jar for preserving; the glass construction of commercial jam jars isn’t meant for repeated use, and the lids will not give you a reliable seal.
When reusing the screw bands that hold the snap lids down, make sure they’re not rusty on the inside or bent out of shape. These can also prevent perfect seals.
As for jars, visually inspect them to make sure there are no dings, nicks or scratches on the inside or outside of the jar. A chip on the rim can prevent a good seal from forming, while other chips in the glass can compromise the structural integrity of the jar. Since the glass is being boiled and subjected to internal pressure, it’s very important to make sure your glass jars are in excellent condition.
Of course, if you’re just using the jar to store dried lentils in the pantry, a used snap lid is fine. If you want to use it for this, mark the top of the lid so you know not to use it again for preserving. Plastic storage lids are also made for this purpose. They are only to be used for storage and not for preserving in a boiling water bath.