That is, to one without a hammer, nothing looks like a nail.
They irony of this is that for one with a hammer, everything looks like a nail.
The real problem is that programmers have spent far too much time worrying about efficiency in the wrong places and at the wrong times; premature optimization is the root of all evil (or at least most of it) in programming. Donald Knuth.
Knowing when to apply something is just as important as know where
A lack of hammer implies only having a screwdriver.
The point is you should have a tool box (and garage and workshop and spare bedroom) full of tools so you have the exact tool you actually need.
What the Phillips head and pozidrive represent in this metaphor are left as an exercise for the reader. As is the adjustable spanner.