If an electrician came to your house, and offered to save you running new wiring by using the fact that the Aluminium Window frames are pretty conductive....
So why do we do the same thing in software?
"I'll just add this feature in here" translates directly to "Man is this ever gonna be a mess in a few years time".
It's an invariant. The words "I'll Just" almost always precede a short term fix that will eventually pile up with all the other "I'll Just" fixes until the whole thing becomes a tangled mess of surprises.
You look in a function and find that it's also doing something else unrelated. "Ah yes, they wanted to be able to do.... so I just added it here"
There's a name for this one, "accidental coupling", and no, that has nothing to do with being drunk in a nightclub. Things get tied together because they happen together by coincidence.
Of course, later someone wants to do them separately. And in code, surprises are almost* never good things.
The PrintInvoice() function also sets the credit limit. Who'd have thought that? "Well we did that because we usually set the credit limit then..."
It was a wrong thing to do, and any and all justifications don't change that. Great, You've just used the Aluminium window Frame to carry the current to the Outside Lights. Nice.
There is a minimum amount of work that is required to do something right. You cannot avoid that work forever. You can put off that work, but when it comes to tidying up all the shortcuts, you may not like the interest and penalties that build up on all the work you should have done up front.
*Almost Never ~ ok, let's just say Never
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