While the body works in a complex way by coordinating all its systems at every moment in our life, it also functions in a simple way: apply a stress, and it responds.
This linear relationship is the very foundation for training adaptations. Tell the body to do something new, and it ups its ante to meet those new requirements.
So why - or better said HOW - does strength make everything else in life easier?
In a study using magnetic resonance imaging (1), researchers found that less muscle mass is used for lifting a given weight as strength goes up.
Consequently, our PERCEPTION of lifting a given weight also decreases.
Isn't it fascinating how simple things can be? You send the body a message of "increased demands", and it carries this message into performing daily tasks.
Now, when and where are we concerned with handling external loads in our life?
Basically ANY TIME we physically interact with our environment. This doesn't just apply to generating force for lifting external objects. It also applies to using our own body weight: running, jumping, sprinting, changing direction, punching, kicking....all the way to the simplest tasks such as getting out of a chair at an old age.
Even the process of building our physique heavily relies on our ability to produce force.
Stronger muscles have more potential to grow as the impact they have to handle - defined by the weight on the bar - is higher than with a lighter weight.
"But why do I need to deadlift 150kg if the shopping bag only weighs two kilos?"
Let's answer this question first: would you rather have thirty thousand our sixty thousand pounds in your bank account?
You might only use a fraction of this - but it's always good to have something in reserve, isn't it?
The same applies to strength:
A "strength reserve" ensures that we stay strong, independent, and active until the last day of our life.
In fact, someone with a 200kg deadlift can produce more force than someone with a 120kg deadlift. That means the stronger person will find things "easier" in life than the weaker person.
So - after knowing all of this -
Why would we NOT train for strength?
Reference:
(1) Ploutz et al., 1994: Effect of resistance training on muscle use during exercise. J Appl Physiol (1985) 76: 1675–1681, 1994.