In the last 3 years in particular, I have been working on my core strength and stretching. It helped my back problems, rehab after back surgery, swimming and stability. The core is the connector between your upper and lower body and should be thought of as a vital link for triathlon.
When you run, you swing your arms to counter to your leg movement. In the middle is the core. If you get out of your saddle while cycling, your hands grasp the handlebars while feet are in the pedals. The middle link is the core. The link between your stroke and your kick while swimming is your core.
As I increased my core strength, I noticed simple functional things in daily life also got easier like bending over, getting up and good posture. These make so much more of a difference now than when I was 18 years old.
Hopefully, you can see that core strength can be important to daily life as well as triathlon.
I used little else besides a physio ball,my body weight and the occasional medicine ball. There are a number of books available but from the ones I have read, I recommend these two:
“The Core Performance” has many exercises I do and it has training regimens for several different levels of athletes. I view the “Complete Book of Core Training” as successfully trying to be just that. It has many many pages of different core exercises with different variations to try.
Don’t forget to check out the short videos on Resources/Core page as well.