Question

why is the ideal gas law useful even though the ideal gases do not exist

Answer 1

Sometimes it's a very close approximation!
An approximations work well enough when they're close!

A numerical approximation is used with \(\pi\). We know \(\pi\) has over like 1million digits, and we'll do calculations with like five. It's an approximation, but it's close.

In physics, sometimes situations are similar enough that we can use an approximations and it's close enough! For example, balls aren't completely round and surfaces aren't completely flat. But we sometimes suggest that they are, as an approximation, because they are close to perfectly round or flat.
Just ask:
What properties does a perfectly flat surface have?
Does this surface have all its properties very similar to all of the properties of the ideal flat surface?

So, \(\rm\color{green}{why~can~we~approximate~a~gas~to~be~ideal?}\)
Well, what properties does an ideal gas have?
Do real gasses ever have very similar properties?