I kind of saw this idea or better yet a foolish notion of the fallacy of 'arguing from ignorance:' "if you can't observe something, science simply says nothing about it," Much of today's physics functions very well 'without seeing it.
The preference for having
theories that are testable -- theories that make predictions on not yet observed phenomena that
can be checked on someday, in time...
...so that it's possible that theory
might be found to be false by observations... --> theories that are "falsifiable" as Karl Popper points out..
The reason this quality of
falsifiability is so valued, prized, is because of the terrible situation when we don't have falsifiability....
Without falsifiability in new theories, then.... as more and more new theories that
cannot be falsified (ever so far as we know) gradually accumulate....first 5, then 10, then 20.... we begin to get over time to a proliferation of new theories, with little or no way to discard any...
So that in time it will become more obvious that such theories have little value, because
one can't know which one might be useful to pursue further development on...
But, in contrast to that bad situation, when a new theory is falsifiable (testable), then we
can try to test it in time, and rule it out
or find it survives a key test and then becomes much more interesting to test further in new ways.
So, falsifiability is very useful/valuable. Without it, we can waste time chasing illusions.