I often find holes wear in my jeans between the legs, so I patch them with fabric on the inside and lots of rows of zig-zag sewing.
It's not a very visible spot so it does not have to be perfect, but it definitely increases the useable life of the jeans considerably.
Darning and patching are popular topics in the raw denim community. One of the draws of raw denim jeans is that they're typically made of heavier weight fabric and don't come pre-faded or pre-shrunk, so with proper care they can last a lot longer than big box fast fashion jeans. They're also usually produced by small makers from all over, so it can be a great way to buy local. Of course the downside is that they're many times more expensive than what you'll find in department stores.
Some denimheads like to get creative with eye-catching thread colors, patch fabrics, and darning patterns. Coupled with the exaggerated fading patterns that can be achieved with raw denim, it's really neat to see!
Edit: spelling