We tend to overlook elbow health until our elbows are injured. Then they take ages to heal. This simple exercise prevents many elbow issues.
The first time I ran across the term "old man elbows", I was in my mid 40's and had been training for a certification that required me to be able to do a lot of pull ups.
A number of the other guys my age and older on the course complained about the injuries they'd been suffering, to which the instructor replied that they were suffering from "old man elbows", particularly prevalent, he said, among older athletes who did a lot of pull ups.
Correctly termed, we're talking about medial epicondylitis (golfers elbow), where the inside of your elbow hurts and lateral epicondylitis (tennis elbow), where the outside of your elbow hurts. Of course, these are chronic (long-term) conditions and you want to avoid getting to the point where they become limitations on your life and training activities.
I never quite got to the point where I was injured, but by the end of that certification, I would certainly have described my elbows as "tweaky".
Of course, being me, I set out to figure out how to prevent this in the future.
This is the exercise I started doing regularly, and I very seldom have any issues with my elbow health anymore. Even across the last few months where most of my own training has been calisthenics-orientated (lots of dips, pull ups, one-arm press ups) and overhead kettlebell military pressing, all of which is particularly taxing on the elbows, tweaky elbows have not derailed my progress.
The simple version, extending fingers against resistance never seemed to do enough to keep my elbows totally happy. Adding in different positions and a bit of movement made all the difference.